Fincher & Thornhill's (F&T's) model predicts that individuals who live under heightened parasitic loads should be motivated to form fewer, more restrictive, and less fluid social relationships, and express higher levels of affect and related gestural behaviors that function to attract and maintain smaller, more intimate social networks. This thesis is contradicted by cross-cultural studies showing that people who live closer to the equator and in warmer climates – conditions that favor parasitic diversity – report higher levels of happiness and confidence, whereas people who live in darker and colder climates report more sadness and worrying behaviors (Agumadu et al. Reference Agumadu, Yousufi, Malik, Nguyen, Jackson, Soleymani, Thrower, Peterman, Walters, Niemtzoff, Bartko and Postolache2004; de Graaf et al. Reference de Graaf, van Dorsselaer, ten Have, Schoemaker and Vollebergh2005; Van de Vliert et al. Reference Van de Vliert, Huang and Parker2004; Kovalenko et al. Reference Kovalenko, Hoven, Wicks, Moore, Mandell and Liu2000; Okawa et al. Reference Okawa, Shirakawa, Uchiyama, Oguri, Kohsaka, Mishima, Sakamoto, Inoue, Kamei and Takahashi1996; Rehdanz & Maddison Reference Rehdanz and Maddison2005). Indeed, expressed happiness is rated among the most preferred characteristics in a potential new friend, and is thus effective for attracting novel/risky relationship partners (Chang Reference Chang2004; Farmer et al. Reference Farmer, Bishop, O'Neal and Cairns2003; Vigil Reference Vigil2007; Xu & Zhang Reference Xu and Zhang2007), thereby increasing the size of one's social network and risk of exposure to foreign pathogens. On the other hand, sadness and worrying behaviors are more effective for inducing solicitous responses from reliable/existing relationship partners (e.g., Kaniasty & Norris Reference Kaniasty and Norris1995; Terwogt Reference Terwogt2002; Vigil Reference Vigil2008). Related incongruities are findings of higher levels of happiness and of confidence, and the formation of larger social networks among more religious and more conservative-leaning people in Western samples (e.g., Keyes & Reitzes Reference Keyes and Reitzes2007; Napier & Jost Reference Napier and Jost2008; Vigil Reference Vigil2010). Other research has failed altogether to find a correlation between sensitivity to pathogen disgust and conservatism (Tybur et al. Reference Tybur, Merriman, Caldwell Hooper, McDonald and Navarrete2010).

An alternative “socio-relational” model for explaining the above findings suggests that people are primed to behaviorally advertise differing fundamental components of their reciprocity potential, or value as a prospective social partner, depending on the opportunity and the effectiveness at using such traits for regulating different types of social networks (Vigil Reference Vigil2009). Some affect behaviors, such as expressed joy and confidence, are functional for demonstrating personal empowerment or one's capacity to reciprocate, whereas other behaviors, such as sadness and worrying, are more effective at conveying the impression of appeasement and vulnerability (i.e., non-threat) and general trustworthiness attributes. Capacity cues (e.g., physical attributes) are more immediately discernable through limited interactions as compared to trustworthiness cues (e.g., interpersonal attributes), which instead require repeated interactions to accurately verify in others. Humans may have therefore relied on the former to regulate larger, more fluid, peer networks that limit the amount of time that can be invested in individual relationships' and relied on the latter to regulate smaller, more intimate social networks that facilitate the opportunity to advertise time-consuming investment behaviors (Vigil Reference Vigil2009). Thus, from a socio-relational perspective, it makes sense that people would be primed to heuristically express more network-aggrandizing capacity cues (e.g., felt happiness) under climatic conditions that facilitate the ability to interact with a greater number of affiliates (warmer climates), and to express network-consolidating trustworthiness cues (e.g., felt sadness) under climatic and topographical conditions that physically limit the ability to interact with others (colder climates; Vigil Reference Vigil2009). Similarly, patterns among conservatives (e.g., higher income, more joy and aggression, and, important here, more peer relationships; Vigil Reference Vigil2010) can be understood from the socio-relational thesis that experiential prosperity precipitates the behavioral advertisement of personal empowerment cues that are effective at regulating larger, riskier social networks (Vigil Reference Vigil2009; Reference Vigil2010).

However, perhaps the greatest limitation of F&T's model is that it cannot currently account for intra-regional variability in assortative sociality, such as the phenomenon of developmental changes and sex differences in many of the phenotypes that the authors describe. For instance, females show higher levels of religiosity, liberal-political ideals, in-group helping (compassion) behaviors, and out-group stigmatization; and they form smaller, more intimate and exclusive social networks as compared with males (Eagly & Crowley Reference Eagly and Crowley1986; Ekehammar et al. Reference Ekehammar, Akrami and Araya2003; Geary et al. Reference Geary, Byrd-Craven, Hoard, Vigil and Numtee2003; Norrander & Wilcox Reference Norrander and Wilcox2008; Rose & Rudolph Reference Rose and Rudolph2006; Stark Reference Stark2002; Walter & Davie Reference Walter and Davie1998; Vigil Reference Vigil2009). Though not addressed by the authors, many of these sex differences can be explained by an evolutionary history of male–male coalitional competition and male-biased philopatry, whereby males tended to remain in closer proximity to their male-kin, while females emigrated into the social networks of their husbands, essentially heightening their risk of parasitic infection (Geary Reference Geary2002; Geary Reference Geary2010; Geary & Flinn Reference Geary and Flinn2002; Wrangham & Peterson Reference Wrangham and Peterson1996). Females have more active immune systems than do males (Bouman et al. Reference Bouman, Heineman and Faas2005; Klein Reference Klein2000; Zuk & McKean Reference Zuk and McKean1996). Thus, some sex differences in assortative sociality and immune functioning are consistent with the parasitic-threat hypothesis that sex differences in societal behaviors and in social networks may be due to ancestral females having been exposed to higher levels of parasitic threat in their ecology.

Still, sex differences in emotional functioning are well established, with males reporting higher levels of empowerment gestures (e.g., inflated confidence), and females reporting higher levels of vulnerability gestures (e.g., sadness, worrying, and pain behaviors; see Vigil Reference Vigil2009). These dimorphisms are accountable by the socio-relational model which predicts that male-biased philopatry increased the benefit for females to form smaller and more protective social networks, to develop higher cognitive thresholds for trusting peers, and to signal higher levels of trustworthiness (e.g., vulnerability) cues to attract and maintain more continuous and reliable relationship partners in the absence of strong (inclusive-fitness) familial bonds. Males, in contrast, having evolved within kin-based communities, would have experienced a reduced benefit to form intimate, time-consuming relationships and a greater cost for advertising trust cues in favor of capacity gestures, which may be more efficient for regulating a greater number of relationships, and thus larger and more functional coalitions (Vigil Reference Vigil2009). Hence, many of the instances of intra-regional assortative sociality that covary with sex can be accounted by a socio-relational model which capitalizes on the differential expression of fundamental social cues for maintaining distinct social networks. It is therefore likely that the selection pressures that each model emphasizes complement each other in their ability to account for both inter- and intra-regional assortative social cognitive/behavioral strategies.