Dunbar’s arguments about how mammals, especially primates and humans, adapt to the challenges of living in groups are both novel and important for understanding human social structure, behavior, and cognition. Nuance to Dunbar’s view can be added, however, by considering the implications of behavioral ecologic and life history theory. In brief, organisms, including humans, adapt to the challenges of their environment by making trade-offs to deal with limited resources, both temporal and energetic (Del Giudice & Belsky, Reference Del Giudice, Belsky, Buss and Hawley2011; Figueredo et al., Reference Figueredo, Vásquez, Brumbach and Schneider2007; Maranges et al., Reference Maranges, Haddad, Psihogios and Timbs2025). Challenges in the environment include harshness (i.e., sources of morbidity-mortality: threat, deprivation) and unpredictability (i.e., stochastic changes in harshness or environment structure). Both evolutionarily and ontogenetically, humans adapt to cues of harshness and unpredictability via suites of physiological, cognitive, and behavioral strategies that make the most of the environment for reproduction, parenting, and investing in one’s group.

These strategies are generally described on a spectrum, from slow to fast. Slower, versus faster, life history strategies arise from more stable, more resource-deprived, less threatening childhood environments and feature later reproductive development and behaviors, fewer offspring, fewer sexual partners, and investing more in those familial and other social partners. On the other hand, those who experience more unpredictable and harsh childhood environments tend to evince faster life history strategies characterized by opposite patterns (Ellis et al., Reference Ellis, Sheridan, Belsky and McLaughlin2022; Figueredo et al., Reference Figueredo, Vásquez, Brumbach and Schneider2007).

Consider the fundamental issue of the infertility trap, which describes a curvilinear, upside-down-U relationship between group size and female fertility. This phenomenon itself cannot be understood separately from behavioral ecology. That is, fertility is also shaped by environmental harshness and unpredictability. Harshness as a lack of energetic resources is associated with delayed puberty/reproduction and fewer offspring. When faced with limited nutrition, energy, and survival resources, the adaptive response is to prioritize somatic maintenance and survival rather than reproduction (Ellis et al., Reference Ellis, Figueredo, Brumbach and Schlomer2009, Reference Ellis, Sheridan, Belsky and McLaughlin2022). In contrast, harshness as threat and unpredictability of resources or environmental structure are associated with earlier puberty and reproduction and more offspring (Belsky, Schlomer, & Ellis, Reference Belsky, Schlomer and Ellis2012; Ellis et al., Reference Ellis, Figueredo, Brumbach and Schlomer2009, Reference Ellis, Sheridan, Belsky and McLaughlin2022). When faced with high levels of threat or unpredictability linked to one’s morbidity/mortality, time and energy would be better allocated toward earlier and greater reproduction rather than one’s future, which is at risk or uncertain. The former strategy entails more temporal and energetic investment in existing offspring, whereas the latter entails less investment in each, balancing energy to environmental demands, which has downstream implications for the proposed structural, behavioral, and cognitive solutions.

With respect to structural solutions, the very size and stability of the group based on grooming cliques and chains will be shaped by harshness and unpredictability. For example, harshness as resource deprivation is associated with being other-oriented (Maranges et al., Reference Maranges, Hasty, Maner and Conway2021; Stephens, Markus, & Townsend, Reference Stephens, Markus and Townsend2007), while harshness as threat is associated with more empathetic concern but fewer social connections (Ceballos & Watt, Reference Ceballos and Watt2023; Maranges et al., Reference Maranges, Hasty, Maner and Conway2021). The harsher (but more stable) the environment, the smaller but more stable the group should be. This facilitates sharing stable, limited resources across the group, and in humans, this is facilitated by more “grooming” (i.e., caring about and investing in close others) but having a smaller network. If there is certainty about resources, people will share their energy and time with their strong ties. However, unpredictable environments are associated with less empathetic concerns and low social support provisioning. Accordingly, unpredictable environments should give rise to more unstable groups, with less “grooming” behavior (i.e., caring about and investing in close others; Maranges et al., Reference Maranges, Hasty, Maner and Conway2021). Indeed, faster, versus slower, life history strategies are characterized by a strategic withholding of investment in family and others in the community as well as fewer tight and loose social ties (Figueredo et al., Reference Figueredo, Vásquez, Brumbach and Schneider2007).

Dunbar also posits that behavioral investment in close social partners is made possible by eating densely nutritious foods, thereby freeing up time and energy for those investments. Harshness and unpredictability are paramount inputs into organisms’ eating behaviors, which become dysregulated by high levels of these environmental challenges. That is, environmental harshness and unpredictability predict modern-day eating disorders, which cluster with fast life history strategies (Leyva & Hill, Reference Leyva and Hill2018; Lie et al., Reference Lie, Bulik, Andreassen, Rø and Bang2021; Luo, Niu, & Chen, Reference Luo, Niu and Chen2020; Nettersheim et al., Reference Nettersheim, Gerlach, Herpertz, Abed, Figueredo and Brüne2018; Ross & Gill, Reference Ross and Gill2002). For example, harshness as threat predicts higher likelihood of engaging in binge/purging behaviors (Lie et al., Reference Lie, Bulik, Andreassen, Rø and Bang2021), which both use up food resources and waste the energy benefits. Thus, unpredictability and harshness interfere with a quintessential behavioral adaptation to group living according to Dunbar and must be taken into account.

Finally, behavioral ecology directly and strongly shapes the cognitive solutions (i.e., self-control, mentalizing, and executive function) facilitated by a larger neocortex (Maranges et al., Reference Maranges, Hasty, Martinez and Maner2022, Reference Maranges, Haddad, Psihogios and Timbs2025; McLaughlin, Sheridan, & Nelson, Reference McLaughlin, Sheridan and Nelson2017; Sheridan et al., Reference Sheridan, Peverill, Finn and McLaughlin2017). Harshness, as both deprivation and threat, and unpredictability undermine development of neocortical systems (McLaughlin et al., Reference McLaughlin, Sheridan and Nelson2017; Sheridan et al., Reference Sheridan, Peverill, Finn and McLaughlin2017), resulting in faster life history strategies characterized by poorer executive functioning and self-control (Davies et al., Reference Davies, Pearson, Coe, Hentges and Sturge-Apple2021; Fields et al., Reference Fields, Bloom, VanTieghem, Harmon, Choy, Camacho and Tottenham2021; Maranges et al., Reference Maranges, Hasty, Martinez and Maner2022). Unpredictability selects for poorer mentalizing capacities, whereas harshness (as resource deprivation) often selects for and slow life history feature stronger mentalizing capacities (Ceballos & Watt, Reference Ceballos and Watt2023; Figueredo et al., Reference Figueredo, Vásquez, Brumbach and Schneider2007; Maranges et al., Reference Maranges, Hasty, Martinez and Maner2022). Variation in these cognitive solutions then accounts for the link between environmental pressures and social decisions about protecting and helping others (Maranges et al., Reference Maranges, Hasty, Martinez and Maner2022). These patterns suggest that the cognitive solutions to group living are strongly affected by the ecology, which should be considered in concert with the inherent stressors of group living Dunbar outlines.

We argue for the importance of applying a behavioral, ecological, and life history theoretical framework to understand the problems and solutions to group fragmentation and the infertility trap. Applying this perspective both complements and extends Dunbar’s work by providing novel hypotheses about solutions to group living. Whether particular solutions will be evinced and efficacious depends on the life history strategy of the group members, which depends on the ecology.