Evolutionary Life History Theory (LHT) emerged in evolutionary biology (e.g., Pianka Reference Pianka1970) to describe the relationship between environmental conditions and reproductive patterns across species. The vast majority of research addressing LHT has been conducted in nonhuman species, and usually makes comparisons between species. In recent years, there has been a growing interest in human life history and life history variation within species (e.g., Heath & Hadley Reference Heath and Hadley1998). LHT holds great promise for promoting the understanding of our own species and integrating findings from diverse research methodologies spanning multiple levels of analyses.
Following the behavioral ecology principle that behavioral strategies related to reproductive success are conditional based on the characteristics of the physical, economic, and social environment (Crawford & Anderson Reference Crawford and Anderson1989), Belsky et al. (Reference Belsky, Steinberg and Draper1991) proposed that the attachment process is an evolved psychological system that evaluates life conditions and chooses reproductive strategies appropriate to the developmental environment. Del Giudice provides an overview of research related to this theory across a variety of perspectives and enhances its precision of prediction by adding insights from sex differences in human reproductive strategies. The target article, “Sex, attachment, and the development of reproductive strategies,” demonstrates the value of LHT as a framework for integrating converging evidence across fields and levels of analyses to result in a more complete and comprehensive understanding of the complex factors underlying human behavioral patterns.
Evolution by natural and sexual selection is the most powerful theory in the life sciences, and in recent decades there has been considerable progress in using evolutionary theory to explain behavior, especially human behavior. E. O. Wilson's (Reference Wilson1975) book, Sociobiology: The New Synthesis, generated considerable political controversy in the heyday of social relativism, where theories describing the biological bases for behavior were interpreted as a threat to the goal of social equality. More recently, “evolutionary psychology” has emerged as the dominant moniker for the study of evolution and human behavior. The added value that evolutionary psychology brings to the study of evolution and human behavior is the identification of proximal mental processes or mechanisms, which guide behavioral strategies (see Cosmides & Tooby Reference Cosmides, Tooby, Hirschfeld and Gelman1994). Darwinian anthropologists and others rightly argue that “evolutionary psychology” may be too narrow as a proper descriptive term. As can be seen in Del Giudice's article, the evolutionary framework crosses traditional disciplinary boundaries and promotes a universally intelligible account that may enhance communication between researchers using quite divergent methodologies.
Tinbergen's (Reference Tinbergen1963) four questions may be used to assess the maturation of explanations for behavior. These questions entail the evolutionary (ultimate) explanations of function (adaptation) and phylogeny (evolution), as well as the proximate explanations of causal mechanisms and ontological development. Del Giudice's overview addresses each of these areas. Del Giudice rightly identifies Darwin's theory of evolution by selection as the only viable account for biological design. He describes how the psychological attachment system is an adaptation to promote reproductive success through the selection of sex-specific reproductive strategies based on the availability of caregivers during development. Biological (including psychological) processes are designed by selection to promote inclusive fitness, and Del Giudice's overview explains why seemingly dysfunctional behavioral patterns are actually useful in promoting an individual's contribution to future generations.
Del Giudice details cross-cultural studies which strongly suggest that the attachment system is a human universal, rather than a product of cultures with particular familiar patterns. In fact, variations in attachment patterns follow the expected direction based on local mating systems and family structures. Del Giudice emphasizes the unique combination of facultative paternal care and cooperative breeding in humans as the key distinction of our species. He notes the existence of adrenal puberty in gorillas and chimpanzees and its absence in other primates and most other mammalian species, as well as the experimental evidence from the manipulation of hormonal levels across a variety of species. However, the phylogenetic context of the attachment system is perhaps the weakest aspect of his overview. This may be easily remedied in part by inclusion of research such as Harry Harlow's maternal-deprivation and social isolation experiments on rhesus monkeys (e.g., Harlow Reference Harlow, Abrams, Gurner and Tomal1964), which were inspired by Bowlby's work on human child institutionalization. A fuller phylogenetic understanding may require further research on species that vary in phylogenetic proximity to humans and on the extent of parental care.
Del Giudice's overview is particularly adept at summarizing proximate explanations. He describes the neuroendocrine bases for the attachment system. Our current understanding of the interplay between psychology and underlying neuroendocrine mechanisms is incomplete, though research is flourishing and likely far from the point of diminishing returns. As models of neuroendocrine systems increase in precision, they will enhance the ability of attachment theorists to specify the causal cascade contributing to patterns of reproductive behaviors.
The theory's depiction of causal mechanisms is properly allied with an account of the developmental processes guiding behavioral strategies. The quality of parental care serves as a proxy for environmental conditions and suggests locally effective strategies. Early caregiving experiences influence attachment styles, which in turn influence reproductive strategies later in life. The LHT emphasis on the importance of the interaction between genes and environment will hopefully further alleviate misconceptions that evolutionary explanations of behavior entail genetic determinism, providing an additional reason to move beyond the unfruitful nature versus nurture debates. The recognition and understanding of adaptive developmental plasticity in Del Giudice's model also provides encouragement and, more importantly, guidance for intervention.
In sum, Del Giudice's account enhances our understanding of the attachment system by integrating findings across several literatures in an evolutionary life history framework. He describes a constellation of phenomena that would be difficult to explain with a competing paradigm or to discount as a “just so” story. His addition of sexual dimorphism in attachment processes proposes further refinement in the description, prediction, and understanding of human psychology and behavior.
