Ellis and colleagues did well in proposing a 2-tiered life history (LH) model that attempts to integrate the energetics and developmental hypotheses. However, their discussion on life history strategies focused narrowly on earlier vs later/faster vs slower reproduction and higher vs lower offspring number. Human life history strategy domains are wide, including physiology, mating, parenting, reward orientation, and cooperation (Baumard, Reference Baumard2019). Therefore, it is unclear whether their 2-tiered LH model could be applied in other social domains (e.g., assertiveness, sociability, and emotional bonding), which were not discussed in the target article. I will expand their discussion by focusing on social behaviors, including shyness and insecure attachment. These social behaviors likely mediate how individuals adapt to environmental stressors, balancing survival priorities with reproductive trade-offs.

Shyness is defined as a form of withdrawn behavior, characterized as the fearful and anxious feelings in novel and strange situations (Asendorpf, Reference Asendorpf1990; Rubin, Coplan, & Bowker, Reference Rubin, Coplan and Bowker2009). It may reflect a conservation-oriented response to environmental threats (Gunther, Youatt, & Pérez-Edgar, Reference Gunther, Youatt, Pérez-Edgar, Schmidt and Poole2020). Shy individuals might conserve energy by avoiding risky social interactions, aligning with delayed maturation, including psychological maturation. It might be explained by the existing interesting results showing that shyness was negatively related to subjective age (Zalk, Kerr, & Tilton-Weaver, Reference Zalk, Kerr and Tilton-Weaver2011). Due to their social fears or poor social skills, shy people might more often be perceived as immature and younger and, thus, treated as such by their peers.

Similar to the countervailing effects discussed in the target article, existing literature also showed opposing findings on the association between shyness and LH outcomes. For example, some research showed that shyness was negatively related to alcohol use (Dubow, Boxer, & Huesmann, Reference Dubow, Boxer and Huesmann2008; Young et al., Reference Young, DiBello, Traylor, Zvolensky and Neighbors2015), but others showed that shyness was positively related to alcohol and substance use (Lewis & O’Neill, Reference Lewis and O’Neill2000; Page, Reference Page1989). For another example, a cross-cultural longitudinal study showed that shy boys married and became fathers later than non-shy boys in both America and Sweden (Kerr, Lambert, & Bem, Reference Kerr, Lambert and Bem1996). Similar findings were found in a German sample demonstrating a delay in establishing a first stable partnership in both inhibited boys and girls (Asendorpf, Denissen, & Van Aken, Reference Asendorpf, Denissen and Van Aken2008). But other research showed that shyness was positively associated with permissive sexual attitudes (e.g., “It is all right for two people to get together for sex and not necessarily expect anything further”), especially for men (Luster et al., Reference Luster, Nelson, Poulsen and Willoughby2013). Perhaps being shy is not completely negative in certain contexts if it keeps individuals from confidently engaging in certain mating activities that might lead to unwanted consequences in energy-limited environmental conditions. In contrast, shyness and its relation to permissive sexual attitudes might be adaptive in response to the ambient cues of extrinsic mortality, as discussed in the target article.

In addition, the integration of attachment theory into the two-tiered life history model might offer a nuanced lens to disentangle how psychosocial cues interact with energetic constraints to shape developmental trajectories. Bowlby’s (1969/Reference Bowlby1982) attachment theory draws heavily upon Darwin’s evolutionary insights. He emphasized that an infant develops a close emotional tie to his or her primary caregiver in order to increase the likelihood of survival. Resources in early childhood take the form primarily of parental care and attention (Chen & Chang, Reference Chen and Chang2012). Secure attachment reflecting the caregiving condition of consistent and responsive parenting may be evolutionarily indicative of abundant resources, low-risk, and predictable environmental conditions. Responding to this condition, secure children explore environments in a relaxed way and have trustful and mutually beneficial attitudes toward people around them. In contrast, insecure attachment, resulting from inconsistent caregiving or resource scarcity during childhood, can be conceptualized as a social-environmental cue signaling environmental harshness and unpredictability (Chen, Reference Chen2018; Ellis et al., Reference Ellis, Figueredo, Brumbach and Schlomer2009). Avoidant attachment reflecting the rearing environments where caregivers are unwilling to invest and even rebuff and reject children’s needs may be indicative of extreme resource scarcity or a harsh environment where survival is at risk (Chisholm, Reference Chisholm1993). Responding to this condition, the avoidant strategy is favored, with pushing toward being self-sufficient and avoiding being abandoned or abused. Anxious attachment caused by caregivers’ unable but not unwilling investment with neglect or inconsistency in caregiving may be indicative of resource shortage rather than depletion in the local environment and an overall unpredictable environment (Chisholm, Reference Chisholm1993). In order to react to this condition, the anxious strategy can maximize the available investment by increasing signals of need and behaving immaturely. The second tier – ambient social-environmental cues, particularly attachment experiences – regulates how these energetic constraints are perceived and responded to. Attachment styles, thus, act as mediators between ambient social-environmental cues and the development of LH strategies, shaping whether individuals adopt a faster or slower pace of reproduction in response to these environmental cues (Chen, Reference Chen2018; Lu et al., Reference Lu, Lansford, Liu, Chen, Bornstein, Skinner and Chang2024).

Empirical research supports this integration of attachment theory within the LH framework. Avoidant attachment could amplify the salience of energetic stress (e.g., food insecurity) as a signal of environmental harshness (Chen, Reference Chen2017b). This heightened perception might prioritize immediate reproductive efforts (e.g., unrestricted sexual attitude) to maximize genetic fitness(Chen, Reference Chen2017a; Chen & Li, Reference Chen and Li2009). Here, avoidant strategies align with faster life history traits, reflecting an adaptive bias toward a faster pace of reproduction in contexts where long-term investment carries high mortality risks. Anxious individuals might delay reproduction to seek safer relational contexts or accumulate resources, reflecting slower life history traits. As Belsky (Reference Belsky1997) hypothesized, anxious attachment is related to “helper-at-the-nest”. Individuals with anxious attachment styles may engage in indirect reproductive strategies by postponing or suppressing their mating opportunities (Chen, Reference Chen2017a). In a short summary, it is possible that insecure attachment may act as a second-tier modulator, refining how individuals interpret and respond to Tier 1 energetic limitations. This integration highlights the need to expand the two-tiered model to include psychological safety as a salient dimension of ambient social-environmental assessment (Chen, Reference Chen2018). Future work should test whether attachment styles mediate the effects of early-life stressors on LH traits, particularly in contexts where environmental harshness/unpredictability and energetic scarcity co-occur.