What do evolutionary researchers believe about human psychology and behavior?

There are important reasons to study what evolutionary-informed researchers believe. First, we repeatedly see misrepresentations or misconceptions of these beliefs in both academic journal articles and the popular press (e.g., Eagly & Wood, 1999; Gould & Lewontin, 1979; Lickliter & Honeycutt, 2003; Park, 2007; Rose & Rose, 2000). Evolutionary scholars have devoted noteworthy effort into correcting such misconceptions, in the 1990s (Kenrick & Simpson, 1997), in the 2000s (Hagen, 2005; Ketelaar & Ellis, 2000; Krebs, 2003; Kurzban, 2002), and the 2010s (Al-Shawaf, 2020; Al-Shawaf & Buss, 2011; Al-Shawaf, Lewis, Wehbe, & Buss, 2019; Confer et al., 2010; Liddle, Bush, & Shackelford, 2011). As misconceptions endure despite numerous attempts at correction, the systematic documentation of beliefs across evolutionary-informed researchers may complement theoretical discussions in clarifying the true nature of the field.

In addition, those who integrate an evolutionary perspective in their research are often viewed uniformly by those who do not. People tend to see the uniqueness in individual in-group members while ignoring the variability across members of other groups (Boldry, Gaertner, & Quinn, 2007). There are, in fact, several topics researched under an evolutionary umbrella that are contentious or controversial, both within and outside the field.

Life history theory is a powerful explanatory framework and is increasingly used in models of, and empirical research on, human psychology and behavior (Del Giudice, Gangestad, & Kaplan, 2015). Life history theory was developed to promote an understanding of the causes and results of the variation in organisms' life cycles. Why do some species take a long time to mature and live long lives, whereas others mature quickly, rapidly produce numerous offspring, and die young? In other words, why do elephants live so long, in largely female kin groups, and produce few young that they nurture for a long time while other species, rabbits for example, have relative short lives, with lots of young that are with their mother for a comparatively quite short time? Initial models proposed that “r-selected” (r is the growth rate of the population) species living in unpredictable environments would evolve clusters of traits associated with rapid and prolific breeding with relatively low investment in offspring (MacArthur and Wilson, 1967; Pianka, 1970). In contrast, “K-selected” (K is the carrying capacity of the population) species living in stable and predictable environments would have lower reproductive rates and longer intergenerational times, invest more in physiological maintenance and care of offspring. Humans as a species are strongly K-selected, more like elephants than rabbits (Hawkes & Paine, 2006; Low, 1998), so initial psychometric measures of human life history variation were proposed to assess “Differential K,” variation within the upper range of the continuum (Figueredo, Vásquez, Brumbach, & Schneider, 2004).

Sæther (1987) introduced the idea of a “fast” to “slow” life history continuum, as biologists recognized that neither the assumption of a trade-off between r and K, nor the association between constant environments and the life-history traits attributed to K-strategists could be justified (Reznick, Bryant, & Bashey, 2002; Roff, 1992). Biologists no longer use the r/K terminology and instead refer to the traits of former r-strategists as fast life history and former K-strategists as slow life history (Jeschke & Kokko, 2009). Psychologists now use the terms fast(er) and slow(er) life histories to refer to individuals with lower and higher K scores, respectively. More recently, both biologists (e.g., Stearns and Rodrigues, 2020) and psychologists (e.g., Del Giudice, 2020; Zietsch & Sidari, 2020) have argued against the existence of a unidimensional continuum in human life history.

Empirical research has identified a second life history dimension related to mating effort that is independent of general life history speed (Differential K) indicators (Richardson et al., 2017). Nested sets of trade-offs in resource allocations are central to life history theory (Roff, 2002). An organisms' resources are limited, and energy used for one purpose cannot be used for another. A life history assessment based on the trade-off between mating effort and parenting effort found two inversely related but distinct dimensions (Kruger, 2017), whereas some research finds that the two dimensions are independent of each other (Valentova, Junior, Štìrbová, Varella, & Fisher, 2020). Network analysis of self-reported life history related measures finds mating effort and parental effort related nodes which are more central than and do not cluster with Differential K indicators (Manson & Kruger, 2022).

The relationship between nature and nurture in influencing behavior has been debated for centuries. And the question, which matters more, nature or nurture, is one discussed in every introduction to psychology course and textbooks. It is also a question debated by philosophers as well as lay-people when they question why someone turns out the way they do. Was it their parents? Their peers? Or was it their genes? In the 20th Century, there were both beliefs that traits such as criminality were inherited, as well as beliefs that people were like an empty slate whose psychological attributes were completely determined by the environments they experienced. Evolutionary approaches to psychology and behavior have been criticized as reductionistic, supposedly arguing that psychology and behavior are determined by genetic inheritance (e.g., Nelkin, 2000). However, evolutionists recognize that very few human psychological mechanisms appear fully functioning at birth and many require input or calibration from the environment (Bjorklund & Pellegrini, 2002). Early childhood and adolescent environments are suggested to play a role in shaping adult responses via conditional adaptations to cope with the environment a child is likely to face as an adult. Although developmental environments clearly have a role to play, the relative contributions of childhood environment, adult environment, and genetics toward adult behavior are still a matter of debate (e.g., Barbaro, Boutwell, Barnes, & Shackelford, 2017; Mustanski, Viken, Kaprio, Pulkkinen, & Rose, 2004; Schlomer & Cho, 2017).

Is the mind a collection of modules solving different specific problems or is it a general problem solver? Historically, there have been two contrasting models for the functional architecture of the mind. “Horizontal” perspectives depict mental processes as interactions between faculties such as memory, perception, and judgment regardless of the content of information. “Vertical” perspectives depict different mental faculties, such as cheater detection, as precisely associated with specific innate neural structures or mental modules. The question of the degree to which the human mind is composed of domain-specific, context sensitive mental modules has been a topic of debate in evolutionary and cognitive psychology (see Barrett & Kurzban, 2006). Much of the early work in this area was shaped by Tooby and Cosmides (1992), who (following but not identical to Fodor, 1983) articulated that our mental mechanisms or information processing systems are functionally specific, processing information relevant to particular tasks, rather than encompassing a broad domain of tasks. The implication of this view is that our mental architecture consists of many specialized information processing systems with their own dedicated function rather than a small number of general-purpose mechanisms that cover a wide range of functions (Cosmides & Tooby, 1994).

Although a pillar of evolutionary psychology for many, modularity is also criticized within the field (e.g., Pietraszewski & Wertz, 2022). One aspect of the modular view of the mind that sometimes causes debate is the idea that such systems imply informational encapsulation, such that a given mechanism has access to only certain informational input (Fodor, 1983; Sperber, 2002), though this input is not a requirement of Cosmides and Tooby's (1994) account (Barrett & Kurzban, 2006). Evidence of such modularity has been found in people with psychological disorders, including those who have experienced the severing of their corpus callosum (Gazzaniga, 1998). Others have argued that humans have a capacity for domain-general processing, pointing out that factor analysis consistently reveals a single “g” factor that explains a substantial degree of variance in task performance (Carroll, 1993). This factor is typically considered a measure of general intelligence, skill across a wide range of cognitive tasks including problem solving and abstract thought (Gottfredson, 1998). There is also evidence of integration across cognitive domains as well as neural evidence suggesting brain regions are activated by tasks in different domains (Anderson, 2010; Anderson & Finlay, 2014). Recently, some studies in language processing have suggested the existence of both domain-specific and domain-general systems (Campbell & Tyler, 2018).

Does evolution occur primarily at the level of the individual (and their associated genes), or could it also happen with groups? Many early biological explanations of social behavior argued that individual behaviors occurred for the good of the species, including those of zoologist Wynne-Edwards (1962) and naturalist Konrad Lorenz (1963). Darwin (1871) suggested synergistic effects between the successes of individuals and the groups in which they lived. Fisher (1930) and Haldane (1932) proposed genetic relatedness as an explanation for altruistic behavior. Lack (1954) determined that birds regulated their clutch size based on the largest number of offspring that parents could regularly provision, rather than what would be best for the group, as was commonly thought at the time. Hamilton, 1964a, Hamilton, 1964b mathematical model of inclusive fitness theory demonstrated that even tendencies for altruistic behaviors could be naturally selected for based on the benefits to genes shared by closely related individuals. Dawkins', 1976 book The Selfish Gene popularized the idea of the gene as the unit of selection and consequently group selection fell out of favor as an explanation for behavior. However, Sober and Wilson (1998) revived interest in group selection (see also Wilson, 1997, Wilson, 2003), sometimes framed as multilevel selection theory, to emphasize that selection simultaneously takes place on multiple levels. Wilson and Sober received support from other prominent academics, such as economist Herbert Gintis (2000).

Krebs (2014) notes that although the existence of altruistic behaviors deriving from group-selection is a theoretical possibility, most scientists believe that few, if any species have met conditions necessary for group selection. Arguments for evolutionarily stable strategies of cooperation to promote the “good of the species” are usually rejected because natural selection operates more effectively within breeding populations than between them (Reeve, 1998). If the tendency to sacrifice oneself for the sake of one's group varies among individuals within groups, those with more selfish tendencies will survive better than their more altruistic neighbors. This trend would lead the group to eventually become more selfish in nature.

Notable Sociobiologist E.O. Wilson caused a stir when he expressed support for group selection during his plenary talk at the 2005 meeting of the Human Behavior and Evolution Society. Nowak, Tarnita, and Wilson (2010) published an article in Nature defending group selection (or attacking inclusive fitness) across species. One hundred and thirty-seven evolutionary researchers co-authored a response article stating that Nowak et al.'s (2010) arguments were based upon a misunderstanding of evolutionary theory and a misrepresentation of the empirical literature (Abbot et al., 2011).

Human sexuality is a popular and controversial topic in contemporary “Western” cultures. Recently, a lecturer in an Ivy League university biology class describing the existence of two sexes and the using the terms “male” and “female” generated considerable debate in academic departments, media, and on-line discussions (Xu, 2021). In The Descent of Man, Darwin (1871) identifies characteristics that influence reproductive competition or success, leading him to distinguish sexual from natural selection (Clutton-Brock, 2017). Darwin outlined how males tend to engage in more intense forms of intrasexual competition for reproductive access and opportunities than females, whereas the latter are more selective of mates. Bateman (1948) showed high reproductive variability in male Drosophila (fruit flies), leading to sex differences in traits associated with winning mating competitions. He concluded that egg production limited female reproduction, but male fertility was unlikely to be restricted by sperm production, and instead limited by access to available females. Parental investment theory (Trivers, 1972) leads to the conclusion that women are the more investing sex because of disproportionate costs related to their gametes, gestation, and postpartum childcare. Further, males profit more readily than females from repeated mating opportunities, males are more eager and less discriminating in mating than females, and male reproductive success is more variable than female reproductive success. However, some have challenged Bateman's (1948) methods, results, and conclusions (Gowaty, Steinichen, & Anderson, 2003; Snyder and Gowaty, 2007). For example, female mating success is tied to their number of partners (Tang-Martinez & Ryder, 2005), but this trend is not as applicable to primates, including humans (Hrdy, 1986). Further, there are advantages in child survival when men supplement a mother's ability to gather resources when she is breastfeeding, the most sensitive period of investment (Marlowe, 2000).

Social psychologists have proposed alternative explanations of human sex differences, most notably social role theory (Eagly, 1987) which argues that sex (or rather gender) differences are a product of the social roles that regulate behavior in adult life. In this framework, there are societal stereotypes about gender based on observations of behavior, such as women having caretaking roles in industrialized societies. Eagly and colleagues (e.g., Eagly & Wood, 1999; Wood & Eagly, 2002) have explicitly challenged evolutionary accounts of human sex differences, including the mate selection criteria observed cross-culturally by evolutionary psychologists (e.g., Buss, 1989).

Wood and Eagly (2002) later developed a “biosocial approach,” in which men's and women's social roles originate primarily in humans' evolved physical sex differences, specifically men's greater size and strength and women's reproductive abilities of gestating and nursing children, which interact with a society's circumstances and culture to make certain activities more efficiently performed by one sex or the other. People carry out gender roles as they enact specific social roles based on their physical capacities and physiological constraints. Eagly and Wood (1999) also call into question the causal direction of the endocrine system and behavioral roles and believe that social influences have separate origins are independent of biological or evolutionary influences.

One question that arises from the debate over the relative roles played by genes and environment in shaping behavior is whether different genotypes are related to individual differences in traits such as personality or intelligence. The radical behaviorism of B.F. Skinner strongly emphasized the environment as a cause of behavior, a perspective that dominated much of 20th Century Psychology. Evolutionary psychology emerged with an emphasis on human universals, in part to emphasize the contrast with blank slate models of human psychology (Pinker, 2002). Some evolutionary psychologists focus on universal human adaptations and assume that there are no individual differences in such adaptations because natural selection would eventually result in a single fitness enhancing mechanism (Buunk & Fisher, 2009). For example, Tooby and Cosmides (1992, p. 38) argue that “heritable variation in a trait generally signals a lack of adaptive significance.” On the other hand, other pioneers of the modern field of evolutionary psychology hold that heritable variation in individual differences may continue to exist because they reflect equally adaptive strategies (Buss, 1989; Gangestad & Simpson, 2000). Although a considerable amount of contemporary evolutionary research focuses on individual differences, there is often disproportional attention to foundational works, especially by those who are not actively researching in the field.

How accurate is the notion that women's emotional states, cravings, or other behaviors change depending on where they are in their menstrual cycle? Fields such as cognitive neuroscience historically excluded women as research participants because it was believed that hormonal fluctuations during different phases of the menstrual cycle would increase variance in results (Beltz & Moser, 2020). In contrast, some social psychologists argued that the notion of hormonal fluctuations across the cycle leading to maladjustment and premenstrual syndromes was a social stereotype that influenced women's self-reported menstrual syndromes (e.g., Wood & Carden, 2014). An adversarial research collaboration established to resolve differences between evolutionary and social constructivist perspectives on cycling effects devolved into separate meta-analyses with contrasting conclusions (Gildersleeve, Haselton, & Fales, 2014; Wood, Kressel, Joshi, & Louie, 2014).

Gildersleeve et al. (2014)’s meta-analysis supports the ovulatory shift hypothesis whereby women have elevated sexual attraction to men with characteristics that signal, via proxy, genetic quality during ovulation. These characteristics are present only when women evaluate men for short-term but not long-term mating. Wood & Carden, 2014; see also Wood et al., 2014) contend that using more precise estimates of menstrual phase reduces these effects. Likewise, Harris, Pashler, and Mickes (2014) argued that the studies on which Gildersleeve and colleagues use in their meta-analysis reflect ‘p-hacking’ (i.e., collecting data or performing statistical analyses to the extent that non-significant findings become significant; Head, Holman, Lanfear, Kahn, & Jennions, 2015). Stern, Gerlach, and Penke (2020) likewise report no ovulatory cycle shifts in women's preferences for men's behavior. Study results also differ in whether affect changes with menstrual phase (e.g., Harvey, Hitchcock, & Prior, 2009; Hengartner et al., 2017; Pierson, Althoff, Thomas, Hillard, & Leskovec, 2021; Van Goozen, Wiegant, Endert, Helmond, & Van de Poll, 1997). Research conducted since the meta-analyses has mixed results, for example a lack of cycle effects on self-reported sociosexuality (Thomas, Armstrong, Stewart-Williams, & Jones, 2021; van Stein, Strauß, & Brenk-Franz, 2019), but significant cycle effects on sexual desire and body image (van Stein et al., 2019), assertiveness, and self-efficacy (Blake, McCartney, & Arslan, 2022).

Many recognize that different ancestral environments created differential pressures that produced anatomical or physiological features adapted to those environments, such as lighter skin among those whose ancestors lived in northern climates with less sunlight. However, there is more debate in terms of differences in behavioral traits or psychological mechanisms. Also, has human evolution ended, perhaps with the rise of agriculture or metalworking technology? Some depictions of evolutionary perspectives, both critical caricatures and supportive simplifications, feature “stone age” people surrounded by modern cultural artifacts. Bowlby (1969) introduced the concept of an environment of evolutionary adaptedness (EEA) for a specific adaptation. Some accounts depict the human EEA as the Pleistocene in general, or more specifically the African Savannah. Although the human EEA is sometimes thought of as a specific constant ecology, with the resulting mental adaptations as fixed human universals, Bowlby (1969) defined EEAs as the set of historically recurring selection pressures that shaped a specific adaptation. Thus, these environments are a statistical abstraction rather than a specific time and place.

Are humans somehow separated from or immune to basic biological processes that affect all other species, as depicted in the Medieval great chain of being and arguments by contemporary theistic creationists? Darwin (1859) noticed wide variation in the morphology of finches on the Galápagos islands, especially in the shapes of their beaks, some of which resembled the beaks of much more distantly related birds. Darwin deduced that all the finches had a common ancestor, but their forms diverged because of the different ecologies (including food sources) on the islands on which they lived. This process of adaptive radiation is central to speciation and is one of the cornerstones of evolutionary biology. However, this cornerstone of evolutionary theory can become controversial when applied to our own species, even among those who teach and study biology at the university level (Kelly & Littlejohn, 2019).

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