This is an open access work published under the terms of the Creative Commons Attribution-ShareAlike 4.0 International License (CC BY-SA 4.0), which allows reusers to distribute, remix, adapt, and build upon the material in any medium or format, so long as attribution is given to the creator. The license allows for commercial use (https://creativecommons.org/ licenses/by-sa/4.0/)

1. Introduction

When the dust of the Second World War gradually settled in Europe and other parts of the world, the United States found itself faced with three options: returning to its old policy of isolationism, relying on its superior power and pursue its foreign policy goals freely and independently, or establishing a new order based on multilateral institutions. Although taking the first and second paths was in line with the rational requirements of realpolitik and being the world superpower, the United States chose to pursue the third way. As a result, many international organizations that were established a decade after the war began to put the “Made in America” label (Hirsh, 2002, p. 31).

Taking the second path, or the “unilateralist temptation”, however, remained the strong alternative. We may define unilateralism as an attempt “to engage the world with as few constraints as possible from norms, treaties, agreements, international organizations, and other countries” (Holsti, 2006, p. 274) or as “unwillingness to work with other countries in solving a problem, and pursuing independent action instead” (Kahn, 2003, p. 548).

There is no doubt that in some periods of the post-WWII era, the US presidents preferred to go alone. Although the period of the early 2000s is often mentioned as an exemplar of US unilateralism (e.g., Leffler, 2004, p. 22), a number of significant unilateral measures had indeed been carried out in other periods as well. The unilateral intervention in Vietnam and the Dominican Republic in the mid-1960s and in Panama in 1989, the withdrawal from the UNIDO in 1995, and the air strikes on Iraq in the late 1990s are examples of such measures. Furthermore, in some periods, even though a few unilateral actions such as the limited unilateral intervention in Lebanon in 1958 and the withdrawal from the ILO in 1977 were carried out, they seem to be rather exceptions to the rule of avoiding unilateralism.

More recently, the US under the Trump administration withdrew from many international treaties and organizations, including, but not limited to, WHO, UNESCO, the UN Human Rights Council, the Paris Climate Agreement, TPP, the Open Skies Treaty, and the Joint Comprehensive Plan of Action (JCPOA). The old US alliances such as NATO became the aim of American leaders’ rhetorical attacks as well, and the usefulness of remaining in such alliances was challenged (Sloan, 2020, p. 39). These actions boldly indicated that Washington wanted to go alone, or at least had insignificant interest in multilateral solutions.

However, unilateralism is generally in contrast with the overall approach of the US foreign policy in the post-war period even regarding military interventions (Finnemore, 1998, p. 181; Pauly & Lansford, 2005, p. 59; Harris, 2018, p. 618) as the US itself made and maintained “the liberal international order” and multilateralism has been “a basic feature of US post-war foreign policy” (Tago, 2005, p. 586). In addition, as indicated by numerous polls, the US general public “strongly” favors a multilateral foreign policy over a unilateral one (Todorov & Mandisodza, 2004, p. 323). Furthermore, unilateralism is considered by some experts as not as “practical” as multilateralism. In the words of Richard Haass (2000, p. 40),

[u]nilateralism has the advantage of minimizing the need for compromise and maximizing speed and ease of acting. But it is also expensive (in both dollars and people) and impractical. Few undertakings can be carried out by the United States alone. Major military operations require overflight rights, access to bases, and contributions of troops and equipment. Unilateral sanctions can easily be circumvented. A world trading system by definition requires the cooperation of others. Supplier clubs designed to curb the proliferation of weapons of mass destruction require near universal membership.

Therefore, it seems that a president’s frequent or bold resort to a unilateral course of action requires an explanation. As a result, in this article, we aim to explain unilateralism in the foreign policies of post-WWII US presidents, i.e., between 1945 and 2020, by examining their cognitive structure or “conceptual complexity”. In other words, we believe that presidents with lower levels of conceptual complexity are more likely to undertake unilateral measures than those with higher levels of conceptually complex presidents.

In a previous study, we compared the conceptual complexities of US presidents from 1945 to 2020 by analyzing a limited amount of data to explain the unorthodoxy in the foreign policy of Donald Trump (Moshirzadeh & Afrasiabi, 2021). The suggested model is not only intended to explain the dominance of unilateralism in some presidents’ foreign policies, but also to predict the probable future cases. Predicting the likely future approach of a US president can provide more time at home and abroad to equip for proper policy-making since unilateral foreign policy affects the policy-making of the country’s allies and adversaries alike, and at the same time, influences American domestic politics. Facing a unilateralist American president, friends find their concerns not to be heard enough and their needs not to be addressed adequately. They are probably encouraged to form new coalitions with other partners, as happened in the case of TPP, or even pursue their own path. Adversaries also should be cautious of the realization of US harsh policies since “passing the global test” would not be necessary anymore and they can no longer count on international institutions to constrain US actions. They would probably go for some form of balancing or deterrence as well. Moreover, taking the unilateral path may also create problems at home relating to the cost of solving global issues by relying solely on American power.

In the following sections, we first present a short review of the existing literature to observe various explanations for unilateralism in the US foreign policy. As we will see, it has rarely been addressed comprehensively and empirically. Therefore, building a new empirical model, able to provide a comprehensive explanation of US unilateralism seems necessary. We suggest that this model can be based on the psychological characteristics of information processing in the minds of decision-makers, especially “conceptual complexity,” which will be elaborated in the second section. In the third section, our research design based on quantitative content analysis for the measurement of conceptual complexity based on the Flesch-Kincaid readability and our method for the measurement of unilateralism will be introduced. We will present and discuss the findings in the fourth section and conclude the paper with the probable implications of the findings.

2. Situational Factors

In general, two categories of factors are considered responsible for the United States’ unilateral approach, both related to the situation in which the presidents decided: international factors, mainly the distribution of states’ capabilities (polarity) and the superiority of American power, and domestic factors such as elections, parties, Congress, and ideology. Although these two groups of works are relatively few in number, they mention various factors that differ in terms of the time period under study and the type of unilateral measures. We will briefly go through them and show why they did not provide adequate answers to the research question.

International factors are rarely overlooked as explanatory factors of both international politics and foreign policy outcomes, and explaining unilateralism has been no exception. Some scholars observe that the end of the Cold War and the elimination of the Soviet threat, make the US reluctant to participate in international arrangements, as a result of which it has returned to its “original” unilateralist position (Harvey, 2003; Ikenberry, 2003; Martin, 1992; Skidmore, 2005). In contrast, the observation that most of the post-Cold War use of force cases carried out by the United States were multilateral, convinced Sarah Kreps that the change in the international distribution of power and the vanishing of the superpower rivalry, which had paralyzed the UN Security Council for decades, will result in a tendency toward multilateralism (Kreps, 2008).

Other international factors, alongside the distribution of power are also studied. In an empirical research, Podliska (2010) examined factors that may lead to the decision to act alone. He argues that if the relative power gap – between the United States and the target of military intervention – is wide and/or the locus of crisis is in the Western Hemisphere and/or national security is at stake, taking the unilateral path is more likely, whereas it is unlikely that a US president decides to take care of a humanitarian crisis unilaterally.

Yet, none of the above international factors could be considered as the cause of unilateral foreign policies, mainly because different approaches were pursued in largely fixed international circumstances. After the end of the Cold War, for instance, the polarity of the international system has remained more or less unchanged or has changed gradually, but each successive US president pursued a completely different foreign policy approach compared to that of his predecessor. Other mentioned factors also seem to be largely case-specific and incapable of being applied to other cases of unilateralism. In addition, some of these factors were related to only one aspect of unilateralism, i.e., military interventions, and other aspects, such as opposition to international institutions were largely ignored.

A considerable number of works discussing unilateralism in US foreign policy were published in the aftermath of the US strike on Iraq in March 2003, and were therefore significantly influenced by it. Among these works, some particularly paid attention to the “ideology” of the George W. Bush administration – the so-called “neoconservatism.” Scholars like G. J. Ikenberry (2003), Sergio Fabbrini (2006), and Jonathan Monten (2007), attributed the “unilateralist turn” in the US foreign policy to the growth of neoconservative ideology in the country’s social and political arenas. American neoconservatives, in contrast to “transnationalist Europe”, do not believe in the effectiveness of multilateral solutions and institutions. Rather, they believe in “American exceptionalism,” which equates the world’s interests with that of the United States. There are also recent works, which, through rigorous and detailed analysis of its roots, identified unilateralism as a stand-alone ideology that casts its shadow over the American foreign policy-making once in a while, although they have not specified the circumstances for its rise and fall (Nichols, 2022).

However, although referring to neoconservative ideology to explain American unilateralism may have valuable insights, it also has serious problems. Particularly, it seems that some characteristics of Bush’s foreign policy are attributed to another characteristic of his administration without clarifying how the latter leads to the former. In addition, even if the dominance of neoconservative ideology in the American decision-making environment was the cause of unilateralism in US foreign policy, such an explanation would only be limited to a certain historical period, whereas there were other unilateral measures in other periods.

Interest groups’ influence is another factor that is taken to affect the adoption of unilateral policies. “Anti-multilateralist” groups who lobby on specific international issues that they find contrary to their interests do not easily get along with US international obligations (Skidmore, 2005; Tepperman, 2004). Economic situation, election cycles, divided government, “lame-duck” presidents who sought lasting legacy while running out of time, and the hesitation of public opinion are some other domestic factors that were mentioned in the literature (Krauthammer, 1990; Potter, 2016; Tago, 2005).

There are also recent studies that address unilateralism in US foreign policy without attempting to explain the approach undertaken by the US government for this unilateralism. Lee (2022) critically revisited the US “war on terror” and American unilateralism in West Asia by examining its legality and consequences. Mahmood et al. (2021) compared the unilateral and multilateral approaches in the US foreign policy in the case of invading Iraq and Afghanistan and concluded that while the latter as a multilateral measure can be justified under international law, the former as a unilateral one cannot.

All in all, situational factors cannot provide a solid basis for an explanation of unilateralism in the post-WWII US foreign policy for a number of reasons. First of all, those who point to relatively stable domestic factors such as interest groups, would not be able to explain the switching from multilateralism to unilateralism and vice versa. Second, if we assume that the decision to resort to force is made first, and then another decision is made about its unilateral or multilateral approach (Podliska, 2010, p. 30), and if, as some scholars say, the president has the upper hand even in the first decision compared to the Congress (Howell & Pevehouse, 2005, pp. 210, 212), he or she will not be in much trouble for the second decision either. Third, after all, these factors have been proposed to explain only one type of unilateral measures, i.e., foreign military interventions, and therefore other aspects of unilateralism would require further explanation.

We do not rule out these factors; in fact, some of these factors contain valuable insights, which can be used in explaining the American unilateralism. However, we suggest that all these factors should go through the “cognitive” filter of the principal foreign policy decision-maker, i.e., the American president, in the first place in order to affect the US foreign policy. In the following section, we will build a model based on “conceptual complexity”, which relates the structure of a president’s cognition to his or her foreign policy approach. It should be noted that regarding the research design, this study is in line with our previous research (Moshirzadeh & Afrasiabi, 2021), which attempted to answer a different question, i.e., the unorthodoxy of Donald Trump’s foreign policy, and analyzed a smaller amount of data.

3. A Model Based on Conceptual Complexity

Due to the shortcomings in the situational level of explanation, in this article, we will focus on the individual who makes decisions. At this level, the central question is: How do particular leaders or decision-makers reach certain decisions? We believe that given the fact that “[s]tate action is the action taken by those acting in the name of the state” (Snyder et al., 2002, p. 59), states’ foreign policies can be studied by examining their leaders’ psychological factors such as cognition. We build our model on the basis of “conceptual complexity” as a cognitive approach to foreign policy analysis. If a person’s cognition is modeled as a cognitive “system,” i.e., a set of specific interrelated elements, then its “complexity” would be determined by the number of elements, and the quantity and variety of relationships between them (Durand, 2013, p. 5). In foreign policy studies, in particular, “the degree of differentiation that an individual shows in characterizing other people, places, policies, ideas, or things” indicates his or her conceptual complexity (Hermann & Hermann, 1989, p. 377). As a result, a person with a high level of conceptual complexity views his or her external world through many different viewpoints. It should be noted that this cognitive feature is usually considered a “fixed personality trait” (Wallace & Suedfeld, 1988, p. 442).

Although not necessarily backed by empirical evidence, it is usually presumed that high and low conceptual complexity have some implications for a leader’s decision-making style. The breadth of the search for information, the expected range of policy alternatives, and the level of decisiveness are among the most noted implications. Leaders with high levels of conceptual complexity tend to eagerly gather information, insist on a wide range of options, and be indecisive. The opposite is true about leaders who are conceptually simple (Preston, 2001, p. 10). We will return to these implications later to validate our results.

We suggest here that a president’s conceptual complexity has strongly influenced whether he or she tends to undertake unilateral measures in foreign policy or avoid them. Precisely, we hypothesize that it is more likely that presidents with a low level of conceptual complexity pursue a unilateral foreign policy in general. Our argument is threefold. First, global challenges are often complex and interrelated (Brands, 2016, p. 118). Global issues not only have many different aspects, but may also spill over into multiple regions at an incredible speed.

Second, multilateral solutions usually address more aspects of the problem. Legitimizing, funding, sharing military bases, exchanging information about the situation on the ground, and sharing responsibility in case of failure are among the aspects addressed in multilateral interventions. Moreover, in the case of international institutions, in addition to involving many states as their parties, they deal with several different aspects of global challenges that usually are not addressed in bilateral agreements. UNCLOS and the Paris Agreement are two illustrative examples. Furthermore, it is worth mentioning that unlike multilateralism, which yields its results in the long run, unilateral measures usually have immediate and short-term benefits, although they might cause harm to long-term interests (Hirsh, 2002, p. 42; U.S. Foreign Policy, 2022). Whether we consider noticing the long-term consequences of an action as a distinct dimension or as the very result of viewing the problem from different viewpoints, it indicates nevertheless a more complex way of thinking about the problem.

Third, as mentioned above, multilateralism is an exhausting and time-consuming process, as it needs all parties to pledge their support (Ryan, 2020, p. 249). As a result, if a leader does not regard those aspects of the issue that require cooperation with others as important, he or she will not have much enthusiasm for that kind of solution. Such a president would prefer to take action without making much effort to garner the cooperation of others and rely solely on the power of his/her country to resolve the problem. He or she not only oversimplifies the problem at hand, but also has a simplistic view of power. As the US has been regarded as the most powerful country in the world at least since the Second World War, many American leaders may believe that their country’s power is sufficient to solve any problem that might arise. In other words, the “limitations” of American power may not be taken into account.

All in all, these premises lead to the conclusion that US presidents with low levels of conceptual complexity are more likely to lean towards unilateralism, while more conceptually complex presidents tend to avoid it. The former, due to their simplistic view, overlook many different dimensions of the issues involved, and may not be concerned about the limitations of the American power. Thus, they opt for unilateral action without much consideration. Furthermore, the eagerness of these leaders for action and their lack of patience for lengthy deliberations (Foster & Keller, 2014, p. 208) further dissuade them from undertaking multilateral action, which generally requires extensive and tedious negotiations. On the other hand, presidents with high levels of conceptual complexity acknowledge that at least some aspects of an international problem do require the involvement of others and are also more mindful of the limitations of the American power due to their attention to detail and their nuanced perceptions of the external world. Therefore, such presidents do not regard pursuing the unilateral course of action as a highly desirable option.

4. Research Design

Testing the article’s main hypothesis involves measuring US presidents’ conceptual complexity as well as the degree of unilateralism in their foreign policy. Both tasks are challenging. Although a well-established technique was employed to measure conceptual complexity, it turned out to be not very promising. In fact, finding an alternative seems not only desirable, but necessary. Measuring unilateralism has not been quite straightforward either, since the rare studies that had attempted to operationalize the concept ended up defining it relatively narrowly. Thus, we need to incorporate other aspects in the concept’s operational definition. We will proceed with elaborating on these two steps.

The well-known method of “content analysis” is usually used to analyze the leaders’ verbal expressions. The American Presidency Project, which started in 1999 at UCSB, serves as an up-to-date comprehensive source of nearly all public statements of US presidents since George Washington, including transcripts of speeches, news conferences, interviews, letters, and other relevant materials. These resources are freely accessible on the project’s website[1]. For the present study, we obtained the transcripts of all news conferences held by 11 post-WWII US presidents from April 12, 1945 (when Harry Truman took office) to January 19, 2021 (the end of Donald Trump’s presidency) that were available on the website of The American Presidency Project.

It should be noted that John Kennedy and Gerald Ford have been excluded from this study due to their relatively short term in office;[2] since it was difficult to draw valid conclusions about the level of unilateralism in their foreign policies, this exclusion seemed inevitable. It should be noted that we had to omit Truman from this study as well, since his responses in news conferences are exceptionally short and unspecific, and they can hardly be considered representative of his thoughts. He responded many times with “no comment” and barely talked about foreign policy in particular. In fact, while other presidents stated about 69 sentences in each news conference on average and none of them said less than 41, there were only 12 sentences in each of Truman’s news conferences, which were also relatively short (13 words on average). Therefore, as measuring his conceptual complexity by analyzing the content of his statements seems impossible, we conducted this study with the remaining 10 presidents.

We extracted only certain parts of news conference transcripts. The foreign policy-related parts of presidents’ responses to the press were extracted to eliminate the probable effect of the context on the author’s conceptual complexity, as some scholars have mentioned such an effect (Conway et al., 2001), although this point seemed to be largely neglected in the numerous studies conducted on conceptual complexity. We omitted the initial statements of news conferences, suspicious of not being “representative,” due to their preparation by presidents’ speechwriting teams. Extremely short sentences (less than 5 words) such as “I don’t know” or “No comment” were also omitted from the analysis as well as quotations, since the former were hardly able to express specific ideas and the latter obviously reflected the original author’s thoughts rather than the president.

As mentioned above, Hermann’s well-established technique for measuring conceptual complexity could not make it through our evaluation, which involved comparing the results of employing the technique on a president’s statements against how he was portrayed. To make this comparison easier, we picked two successive and yet quite dissimilar US presidents based on their portrayal: Barack Obama and Donald Trump. Whereas psychologists believed that Trump “seems unable to appreciate the complexity of either the external world or his internal life” (Frank, 2018, pp. 14, 142), they praised Obama for his “clear thinking” (Décosterd, 2010, p. 101) and even evaluated his thoughts as the most complex among the US presidents because of his frequent reexamination of already expressed ideas from multiple viewpoints (Landau, 2010). However, their measured conceptual complexities using Hermann’s technique were reported equal by two works; both used the ProfilerPlus software to analyze spontaneous remarks, e.g., interviews and news conferences in which they participated during the entire period of both presidents in office (Mahar, 2017; Thiers & Wehner, 2022).

These not-so-promising results convinced us to seek alternative measurement techniques. Readability scores could be such alternatives since they tend to measure the “comprehensibility” of texts and we suppose that complex texts are often less comprehensible, whereas simple texts are usually easy to understand. The readability score that is used here to measure US presidents’ conceptual complexity is the well-known Flesch-Kincaid score. Aiming to introduce a formula whose results correspond more to the real-world competencies of individuals, J. P. Kincaid modified the coefficients and constants of the Flesch formula for the US Navy use (Kincaid et al., 1975). The result of applying the formula to a given text is the educational grade level that is necessary for comprehending it.

The modified readability formula, like the original one, includes two parameters extracted from the text: average sentence length (the number of words per sentence) and average word length (the number of syllables per word). Because

[t]he linguistic measures which have been found to have greatest predictive power are word and sentence length… indicators of semantic and syntactic sources of reading difficulty. In English word length is associated with precise vocabulary, so a reader must usually make extra effort in order to identify the full meaning of a long word, simply because it is precise. Long sentences nearly always have complex grammatical structure, which is a strain on the reader's immediate memory because he has to retain several parts of each sentence before he can combine them into a meaningful whole. (McLaughlin, 1969, p. 640)

In fact, “precise” words and long sentences with “noun modifiers, dependent clauses, nominalized verbs, deletions in coordinate clauses, appositives and clauses used as subject” (Glazer, 1974, p. 647) tend to convey more nuanced and differentiated meaning, corresponding to higher conceptual complexity.

All in all, the overall procedure of measuring presidents’ conceptual complexity using the Flesch-Kincaid technique is as follows: First, all foreign policy-oriented responses in every news conference that is held during each year of a president in office will be aggregated, which will result in a giant text. Counting syllables, words, and sentences will be the next step. Finally, we will calculate the Flesch-Kincaid score for each year and the overall conceptual complexity of a president will be the average of these annual scores. Other than the first step, i.e., extracting a specific type of response from news conference transcripts, all steps will be performed by a software program that is developed by the article’ second author and hence guarantees 100 percent reliability for the measurement.

On the validity of the Flesch-Kincaid score, we need to seek more supporting evidence. At the very least, this alternative technique should do what Hermann’s technique was unable to do: distinguishing Obama and Trump based on their conceptual complexity. As Table 1 indicates, this time the difference is quite evident.

In addition, we will examine the comments on each president’s decision-making style and compare them against the implications that are expected of a political leader with the measured level of conceptual complexity. Finding affirmative observations, therefore, will be counted as supporting evidence for the validity of the Flesch-Kincaid score. We will look for such observations in the interviews and memoirs of the president’s inner circle and also in what analysts reported.

We also need to measure the level of unilateralism for each president. There were extremely few works in which unilateralism was operationally defined. Others either generally took the concept for granted (see, for instance, Brooks & Wohlforth, 2005; Johansen, 1986; Kane, 2006; Maynes, 1999) or while addressing the challenge of defining it, failed to provide an operational definition (e.g., Ikenberry, 2003; Malone & Khong, 2003; Skidmore, 2005). Among those works that defined unilateralism operationally, which were albeit solely concentrated on studying the use of force, the work of Atsushi Tago (2005) is noteworthy. He, building on ideas from other scholars, distinguished between “procedural” and “operational” unilateralism/multilateralism; by the former he meant authorization by an international organization, and by the latter, he referred to “unified” or “joint” command of a military operation. However, this distinction does not seem helpful since there were extremely few cases of merely procedural multilateralism (Tago, 2005, p. 591), and also operating under a joint command per se does not indicate multilateralism, as the command in the Iraq War, as a well-known case of unilateralism, was shared by coalition countries (UK House of Commons Defense Committee, 2006, p. 6). Therefore, we choose not to distinguish two types of unilateralism/multilateralism though we use the idea of IOs’ authorization in our definition.

Moreover, we try to slightly broaden the scope of unilateral behaviors by taking into account the opposition to international institutions expressed in the form of withdrawal from international organizations and/or multilateral agreements. By incorporating this kind of behavior in the operational definition of unilateralism, we present a more balanced and comprehensive approach by considering the long-term challenges that international institutions address in addition to short-term crises that military interventions attempt to resolve.

There are two points regarding unilateral foreign military interventions. First, by foreign military intervention, we mean “the dispatch of national armed forces to another sovereign state in an attempt to influence political, economic, or social conditions in the target country” (Pickering & Kisangani, 2006, p. 363). Thus, behaviors such as rescue operations, peacekeeping, retaliation, fighting against terrorism, supporting coups, dispatching advisors, and propaganda are not considered instances of intervention, since they lack at least one of the two elements. Second, as some other researchers did (see for instance Kreps, 2008, p. 574), we consider the authorization of international organizations as the criteria that distinguishes a multilateral intervention from a unilateral one. The success in securing such permission not only implies the company of other states, but also indicates the “institutional” support for an intervention.

Both unilateral interventions and withdrawals from international institutions are fairly attributable to the US president. He is the only US official that can constitutionally dispatch armed forces (Howell & Pevehose, 2005, pp. 212-214). On the withdrawals, even though the US Constitution is generally silent, the political practice of the post-WWII years only comprises the instances of withdrawal undertaken by the decision of the president (Hessel, 2016, pp. 2396, 2409). We attribute each intervention or withdrawal to the president who ordered the action, regardless of the number of years it lasted or even if a new president took office.

In order to distinguish between the actions at different levels of importance, we categorize each action as either “major” or “minor.” Major actions add a score of 1 to the overall score of a president’s unilateralism, whereas minor actions add 0.5. Categorizing interventions is done based on parameters such as the type of operation (aerial or ground), the number of forces involved, and the duration of the intervention. Furthermore, a withdrawal’s importance is determined by how old the US membership in the organization is, whether the agreement was ratified or not, the number of parties, etc. The sum of these scores will indicate a president’s unilateralism.

5. Findings and Discussion

The results of measuring the conceptual complexities of the 10 post-WWII US presidents are presented in Table 2. The measured values are between 5.34 (for Trump) and 12.17 (for Nixon), the range is 6.83, the average conceptual complexity is 9.86, the standard deviation is 2.03, and the median is 10.22. Some presidents seem rather similar to each other, while others are relatively outliers. Carter and Obama are very close and Nixon is slightly above them. Bushes are also at nearly the same level of conceptual complexity and Trump is at the lowest level. We ran the Shapiro-Wilk Test using SPSS to determine whether the obtained results were normally distributed. The result was 0.303 which is higher than 0.05 and therefore the non-normality of the results was rejected.

Since the conceptual complexity for each president is obtained by calculating the average of his annual scores, Table 2 contains the corresponding standard deviation (SD) as well. As the table indicates, except for Johnson and Eisenhower, the SD for all other presidents is lower than 0.5, and even taking into account these two presidents would raise the average SD only to 0.46. Relatively low SD in the results indicates the general stability of a president’s conceptual complexity over time.

As it has been demonstrated in Chart 1 and Chart 2, the annual score for each president in different years always revolves around one of these four levels: 6, 8, 10, or 12. The pattern of swinging around these levels, however, is not always clear. Nevertheless, even in the cases of presidents with relatively unstable conceptual complexity like Johnson and Eisenhower, the conceptual complexity of a president never reached the upper level or fell down to the lower one. This can be understood as the “second type” of reliability regarding the stability of a measured variable across time (Hermann, 2008, p. 164). Such stability enables analysts to predict a president’s behavior based on his or her conceptual complexity measured in a limited time, even before sitting in The Oval Office.

Before proceeding to report the results of measuring the degree of unilateralism, we would like to compare the observations regarding the presidents’ decision-making styles with the implications of the conceptual complexity level at which each president was located, particularly the broad or limited search for information. The implications of higher levels of conceptual complexity were generally evident in Eisenhower, Nixon, Carter, and Obama’s decision-making style. In the case of Eisenhower, as his national security advisors recalled, the importance he attached to the NSC and Cabinet meetings as an arena for free “give and take” among his subordinates was enormous (Burg, 1979, pp. 16-17; Luter, 1977, p. 18), and this was clearly a sign of his inclination toward gathering information and examining alternatives. Unlike his former boss, Richard Nixon was not a man of crowded meetings; nevertheless, he was no less greedy for collecting information. As Henry Kissinger noted, Nixon forced his NSC machine to “elicit the best thinking within the government and to define the range of choices available to the president” (Kissinger, 1982, p. 414) and he also vehemently refused to be kept uninformed by the bureaucracy of other possible alternatives (cited in George, 1972, p. 754). Jimmy Carter, his successor, also had reportedly a “habit” of asking for the fifth choice, if he was presented with four (Moens, 2021, p. 37). He was also well-known for his “appetite” and even “obsession” with the details of issues (Hess & Pfiffner, 2002, p. 123). The broad search for information was evident in Obama’s decision-making style as well. His national security advisor recalled that he asked not only for senior officials’ opinions in the meetings, but also “the experts sitting along the walls” (Rice, 2019, p. 388). On surging of troops in Afghanistan in 2009, his national security team assembled at least nine times to look at the issue “from every conceivable angle” (Clinton, 2014, pp. 119-120).

We expect the observations about the presidents in mid-levels of conceptual complexity to be mixed. That is, Johnson, Reagan, and the Bushes are expected to be neither eager for information nor ignorant of it, but somewhere in between. Johnson’s hate of the NSC and Cabinet meeting and, at the same time, his willingness to consult rigorously with a few of his closest advisors during “Tuesday lunches” indicates his mixed approach toward information gathering (Mulhollan, n.d.-a, p. 15; Mulhollan, n.d.-b, p. 31). In the case of Ronald Reagan, while there are several reports of his disinterest in “details” (see Aberbach, 2008, p. 203; Newmann, 2004, p. 281; Pach, 2003, p. 97; Pfiffner, 2013, p. 87), there are also observations of his inner circle that indicate the opposite. They recall his eagerness to discuss high priority issues, strategies, and philosophical points, as well as issues that were related to topics such as religion and human rights (Kessel, 1984, p. 255; Knott, 2005, pp. 4, 18; Knott et al., 2005, p. 31). The same can be said more or less about Clinton’s decision-making style: several close observers pointed out his curiosity and “appetite for discussion,” which apparently led to “endless” meetings on foreign policy issues (Foley, 2013, p. 347; Riley, 2016, pp. 353, 362; Young et al., 2014, p. 34). Other reports, however, indicated that only “some” decisions entailed prolonged deliberations (Young et al., 2014, p. 36), and although lengthy discussions took place in his presence, he did not necessarily participate in them (Riley, 2016, p. 349). It was believed that Clinton was never like Carter in relation to details (Riley et al., 2014, p. 35).

Bushes were known to be “gut” decision-makers. Brent Scowcroft, his national security advisor believed that Bush 41 made decisions intuitively and based on what he “felt” to be right (Crabb & Mulcahy, 1995, pp. 254-255; Zelikow & McCall, 2020, p. 108). He also apparently spent limited time on information search, as his Chairman of the JCS recalled that Bush “listen[ed] carefully, not necessarily taking a long time” (Riley & Strong, n.d., p. 12). In the case of his son, Bush 43, although decision-making by instincts was also frequently reported (Matthews, 2017, p. 28; McClellan, 2008, p. 145; Langston, 2007, p. 167), the reports on the matter of information search were highly contradictory. There are people who assess him as a person without “intellectual curiosity” (Burke, 2007, p. 184; McAdams, 2011, p. 38), while others observed Bush’s “deep dives” into the issues in order to receive information on the details (Nelson & Engel, 2019, p. 77).

Finally, reports on Trump’s decision-making style were consistent with what we expected based on his low level of conceptual complexity. The observations generally indicated that he largely disregarded information, whatever the source was, and whoever presented it (Abdiel, 2020, p. 19; Toosi, 2019). John Bolton, his national security advisor, did not evaluate the intelligence briefs as very fruitful since “much of the time was spent listening to Trump, rather than Trump listening to the briefers” (Bolton, 2020, p. 89). He also rarely read PDBs and expected the information he received to be as much concise as possible (Burke, 2018, p. 656). A journalist, who interviewed several White House officials and staff, concluded that “not only didn’t he read, he didn’t listen. He preferred to be the person talking. And he trusted his own expertise—no matter how paltry or irrelevant—more than anyone else’s” (Wolff, 2018, p. 114).

In order to test the relationship between conceptual complexity and unilateralism, we should have the results of measuring the dependent variable as well. These results are presented in Table 3, which also comprises a brief summary of the unilateral actions that make us assign a certain level of unilateralism to a president.

It seems that while certain unilateral measures such as Johnson’s intervention in Vietnam or Trump’s withdrawal from WHO do not need further explanation, for other measures such as Clinton’s withdrawal from UNIDO detained explanations may be necessary. Moreover, Nixon and Obama’s zero scores might seem surprising and thus we should also discuss these cases in more details.

We will first examine the measures that were assigned a 0.5 score. In the case of Eisenhower’s intervention in Lebanon, Johnson’s intervention in the Dominican Republic, and Bush 41’s intervention in Panama, although the dispatch of ground troops to a foreign country in order to influence the political, economic, or social circumstances in target countries did take place and there was no prior authorization of international organizations, only approximately 14,000 (Little, 1996, p. 27), 23,000 troops (Gleijeses, 1978, p. 258), and 24,000 troops (Maechling, 1990, p. 121) participated in these interventions respectively. Moreover, the duration of these operations was relatively short, i.e., approximately a few months (Gandzier, 2006, p. 359; Pear, 1990; Ringler & Shaw, 1970/1992, p. 45). Moreover, intervention in Iraq ordered by Clinton was carried out solely in the form of an air campaign and in just a few days (Hendrickson, 2002, p. 321; Krisch, 1999, pp. 64-65), which was not authorized by any international organization.

Some withdrawals were also scored 0.5. Reagan’s withdrawal from the compulsory jurisdiction of the ICJ in 1984 seemed more like a minor measure since only 26% of UN members and no P5 state accepted it at the time (Kempster, 1985). Bush 43’s withdrawal from the optional protocol to the Vienna Convention on Consular Relations concerning the compulsory settlement of disputes is considered a minor measure because of a similar reason (U.N. Secretary-General, n.d.). His withdrawal from the Kyoto Protocol on the UNFCCC and from the Rome Statute of the ICC was also evaluated as such because the US had recently signed them and had not yet joined them officially. Concerning Trump’s withdrawal from the Paris Agreement, TPP, and JCPOA, since the US had not ratified the agreements, same measurement applies. Finally, the argument on the limited number of member states also applies in the case of his withdrawal from the Open Skies Treaty (Brookes, 2020).

The major withdrawals generally need no further explanation. Pulling the US out of ILO, UNESCO, WHO, and the UN Human Rights Council were obvious major attempts due to these institutions’ vast membership as well as the long history of US membership. The same is true in the case of UNIDO. In fact, although this withdrawal seems not to have been important to attract the attention of scholars, the very same reasons convinced us to categorize it the same way.

The absence of some of the famous American foreign operations in Table 3 may seem surprising. This is because based on the definition provided, those operations were not considered as interventions. An example is Nixon’s invasion of Cambodia in 1970, since its declared objective was destroying the “sanctuaries” of North Vietnamese forces in Cambodia (Clymer, 2004, p. 8; Siniver, 2008, p. 71) rather than influencing Cambodia’s domestic circumstances. In fact, the measure is usually described as a part of the Vietnam War (Clymer, 2004, p. 41; Logevall & Preston, 2008, p. 12; Schmitz, 2014, p. 86; Siniver, 2008, p. 4). Obama’s air campaigns in Iraq and Syria should not be evaluated as interventions either, as their declared objective was defeating the ISIS and did not directly concern the political, economic, or social circumstances in the two countries (Gross, 2017, pp. 249-251). Put another way, based on the definition, any probable changes in the situation in Iraq and Syria, however profound, were unintended consequences of the strikes, and therefore, the strikes cannot be categorized as interventions.

Moreover, some other measures were not evaluated as unilateral ones, though they met the requirements of the definition, because of the authorization or official support of an international organization. Reagan’s invasion of Grenada in 1983 was indeed an intervention; yet, as an international organization (i.e., OECS) officially invited the US for intervention (Brands, 1987, pp. 613-614) the intervention was not considered unilateral. The same is true about Bush 41’s intervention in Kuwait (Persian Gulf War) in 1991 and in Somalia in 1992, as he succeeded in securing the permission of the UN Security Council (Hess, 2009, p. 183; Recchia, 2020, p. 352). Clinton also intervened in Haiti in 1994, which was authorized by the Council (Smith, 1995, pp. 57-58), in Bosnia in 1995 and in Kosovo in 1999, both in the form of NATO’s air campaigns (Papayoanou, 1997, p. 109; Walt, 2000, p. 68). Thus, none of his several interventions can be considered unilateral. Although Bush 43’s intervention in Afghanistan did not officially take place under NATO’s flag, the organization’s historical decision to invoke Article 5 to support the United States’ measures in response to the 9/11 attacks (Harsch, 2011, p. 9) seems like a clear authorization. Finally, NATO’s air campaign in Libya in 2011, which the US “led from behind” (Kaplan, 2016, pp. 48-49), was not a unilateral intervention since not only did it take place under NATO’s flag, but it also had the permission of the UN Security Council (Adler-Nissen & Pouliot, 2014, p. 902).

With the results of measuring the two variables at hand, we can now examine their relationship. These results can be seen side by side in Table 4. In order to measure the correlation, we use Spearman’s rank correlation coefficient, which is a well-known and widely-used measure of correlation for ordinal variables. It yields 0.747. If we consider α = 0.01, for a sample of size 10, the coefficient should be greater than 0.746 for the correlation to be evaluated as statistically significant (one-tailed test). Since this is the case here (p = 0.007), the correlation is established and the null hypothesis would be rejected.

An interesting question would be whether conceptual complexity is correlated with either element of unilateralism individually, i.e., interventions and withdrawals from institutions. Table 5 demonstrates the related scores. We can again answer the question by calculating Spearman’s coefficient for the two constituents separately. The coefficient is obtained at 0.413 (p = 0.118) for interventions and 0.488 (p = 0.76) for institutions’ withdrawal. These values are not greater than 0.746; nor are they greater than 0.564, which is the threshold for α = 0.05. As a result, the individual correlations of conceptual complexity with neither of the two constituents of the dependent variable are statistically significant. This was rather expected, since we can only expect presidents with lower levels of conceptual complexity to choose the unilateral course of action in general, but it cannot be known what specific policies it entails.

Is there any historical evidence that could demonstrate the link between different levels of conceptual complexity and their corresponding unilateralism levels? The answer is yes, but on the basis of only a few instances. The reason is that this type of evidence can only be found in the final stages of decision-making, which usually take place in private, where very few reports might leak out.

The first instance related to Eisenhower’s policy toward the Indochina crisis, where he was presented with a unilateral option. In an NSC meeting, he examined the options from different viewpoints. He expressed that a unilateral intervention might lead to a general war with China and the USSR, force the US to take the role of the world’s “police”, lead to facing the accusation of “colonialism” and “imperialism,” endanger Korea, and not be feasible due to the lack of sufficient American forces to substitute the French (Memorandum of Discussion, 2010). To our knowledge, these considerations convinced Eisenhower not to unilaterally intervene in Indochina. Similarly, Obama refused to undertake a similar measure in Syria, since he evaluated the situation as “risky” (Kaplan, 2016, p. 53). In an NSC meeting, in response to those who compared the situation in Syria with that of Libya, which led to American participation in NATO’s intervention, he clearly enumerated the differences (Kaplan, 2016, p. 50).

On the contrary, Johnson reportedly had a simplistic view of the situation in Indochina as well as that of the American power. Overlooking the complexities of Vietnam, unrealistic optimism about victory by using military forces, no differentiation between the containment strategy carried out in Europe and Asia, and preference for short-term interests were indicators of such a view (Preston, 2012). In the case of Trump’s withdrawal from JCPOA in 2018, he reportedly found the deal to be a “bad deal” that was unfixable and should have been completely replaced since the “whole deal was based on lies, Iran had played the United States for fool” and was “making too much money” (Bolton, 2020, pp. 69, 71, 74). It is not hard to see that such a simplistic and solely economic-driven looser-winner worldview led him to withdraw from the deal.

One last thing to examine here is the exception of Bush 41, whose mid-level conceptual complexity is not consistent with his low level of unilateralism. We suggest that this inconsistency should be attributed to the exceptional international environment in which he decided. Bush was working with “the most accommodating Soviet leadership ever” (Onea, 2013, p. 48) and he was able to obtain Gorbachev’s support in the Gulf War with no extraordinary effort. This exceptional situation has rarely been given to any other American president. Perhaps the only similar case in post-WWII was the Soviet Union’s absence from the Security Council at the time when North Korea invaded South Korea, in which Truman exploited and secured the UN Security Council’s authorization (Goodrich, 1953, p. 92). Once again, the extraordinary situation at the end of the Cold War made obtaining international support rather easy. Otherwise, Bush’s interventions would probably be unilateral, as he acknowledged that the lack of UN SC’s authorization would not prevent him from intervening in the Persian Gulf region (Podliska, 2010, p. 41). It seems that the exceptional “unipolar moment” resulted in what we observe in the case of Bush’s foreign policy.

6. Conclusion

Explaining foreign policy has long been a battlefield of two different camps: those who refer to situational factors, such as international distribution of power and domestic politics, and those who emphasize psychological factors in their explanations. We believe that both approaches have their merits when we face different research questions. In this paper, we argued that in the case of unilateralism in US foreign policy, the latter has more explanatory power. The results suggested a strong correlation between the degree of unilateralism in a US president’s foreign policy and his degree of conceptual complexity, i.e., how much a decision-maker perceives his or her world as complex, nuanced, and multi-dimensional. We found that for 9 out of 10 post-WWII US presidents, there is a statistically significant relation between the two variables and illustrated their causal link. Numerous presidents and the long period of time under study, i.e., nearly 75 years, provide a solid groundwork for theory-building in studying US unilateralism.

In addition to introducing a new and powerful explanatory variable for unilateralism in the US foreign policy, this paper has an important contribution in significantly improving the measurement of unilateralism. Prior to this, measuring the degree of unilateralism was largely limited to instances of the use of military force abroad, i.e., foreign military interventions, while the general concept of unilateralism in foreign policy usually incorporated other types of foreign policy behavior, such as opposition to international institutions. Trying to fill that void, we took into account instances of withdrawal from international organizations and agreements in addition to foreign military interventions, and showed that this more comprehensive variable has a strong correlation with conceptual complexity as a holistic psychological variable.

This paper was in line with our previous work regarding the innovative usage of the Flesch-Kincaid readability index to measure US presidents’ conceptual complexity. While we had intended to answer a different research question and analyze a significantly smaller amount of data then, the current paper reinforces the idea of using the index to measure conceptual complexity. As we have shown, the results were consistent with the reports of presidents’ inner circles.

We suggest that conceptual complexity can also be used to predict the foreign policy approach of a future US president. The straightforwardness of measuring the variable by calculating the Flesch-Kincaid index for spontaneous verbal materials enables scholars to easily predict the relative degree of unilateralism in a US president’s foreign policy. As adopting a unilateral or multilateral approach by a president may affect both US friends and adversaries, using this method can reduce the level of uncertainty regarding the potential future policies of the United States.