According to Del Giudice's model, sex differences in insecure attachment orientations should emerge most strongly when individuals are exposed to moderate levels of stress during development. In such environments, males should be more likely to develop avoidant attachment orientations, and females ought to develop anxious/preoccupied orientations, especially from middle childhood extending into early adulthood. There are two potential problems with this claim. First, valid attachment measures have proven difficult to develop for middle childhood (Kerns Reference Kerns, Cassidy and Shaver2008), which may partially explain the dearth of studies focusing on this age group. Second, gender differences are rarely found or are quite small in adult samples, regardless of whether self-report or interview measures of attachment are used (Crowell et al. Reference Crowell, Fraley, Shaver, Cassidy and Shaver1999/2008). It is not clear how Del Giudice's model reconciles these issues.
When contemplating how stress impacted social development in the environment of evolutionary adaptedness (EEA), one must consider the different sources of stress our ancestors faced and which specific sources were most strongly associated with certain environmental risks, given humans' comparatively long life histories. According to life history theory (Kaplan & Gangestad Reference Kaplan, Gangestad and Buss2005), organisms make trade-offs between current versus future reproduction, quality versus quantity of offspring, parental investment versus offspring genetic quality, and reproduction versus survival over the lifetime. Different sources of stress might have had unique implications for certain life history trade-offs and, ultimately, the adoption of certain adult reproductive strategies.
Four prevalent environmental threats during evolutionary history were predation pressures, the availability of food, the prevalence of diseases, and intergroup or intragroup conflict (Simpson & Belsky Reference Simpson, Belsky, Cassidy and Shaver2008). Each type of threat could have produced a unique source of stress that affected how life history trade-offs were made. For example, if diseases were prevalent in the local environment, most men might have placed more weight on either offspring quantity or their genetic quality, contingent on other mating factors. Most women, however, should have emphasized offspring genetic quality on account of the more limited lifetime reproductive capacity of women and the need to bear the most disease-resistant offspring. These trade-offs may have oriented both sexes toward greater avoidance, which might have facilitated greater male promiscuity and more male–male intrasexual competition (Schmitt Reference Schmitt2005b). Women should have been more strongly attracted to mates who displayed better health or more viability, and women should have competed more intensely to reproduce with these men, putting less emphasis on paternal investment.
In environments characterized by intergroup conflict, many men may have shifted to a shorter-term, higher-quantity mating strategy, given the greater risk of death in ancestral men (Cronin Reference Cronin1991). Most women, however, may have worked to keep their mates invested to secure more protection for themselves and their offspring. Adopting an avoidant orientation might have been the best way for most men to increase their fitness in these environments, whereas an anxious/preoccupied orientation might have been the best strategy for most women. The main point is that different sources of stress might have shifted men and women in the same or in different directions with respect to adult attachment orientations and reproductive strategies, even if the absolute levels of stress experienced during development were similar.
Although the purpose of the target article is to present an integrated life-history-based evolutionary model of reproductive strategies, relatively little is said about how life history trade-offs might intersect with proximal (current) trade-offs. In section 6.3.1 of the target article, the author notes that “women cannot shift the balance between parenting and mating effort as easily as men.” This is precisely why ecologically contingent evolutionary models of human mating such as the Strategic Pluralism Model (SPM; Gangestad & Simpson 2000) have been developed.
Rather than viewing human mating in terms of how an individual's history results in specific adult reproductive strategies, SPM proposes that women evolved to make mating decisions on the basis of the nature of their current local environments. Specifically, women should place relatively greater weight on men's viability (i.e., their health, vigor, and ability to withstand diseases), especially in pathogen-prevalent environments. However, they should place greater emphasis on men's ability and willingness to invest in themselves and their offspring when local environments require more biparental care. Indeed, it is precisely because women cannot easily “shift the balance” between parenting and mating effort that they should have evolved to select mates by making judicious trade-offs between these mate attributes. SPM, therefore, explains how and why women evolved to make the best of one “constraint” imposed on them by nature – the unique way in which they reproduce. A complete evolutionary account of human mating must consider how and why both ontogenetic (life history) and proximal (current environmental) factors led both sexes to make adaptive decisions with respect to the allocation of mating effort versus parenting effort.
How might life-history and ecological-contingency models intersect? An individual's developmental history could set thresholds for judging the acceptability of a mate's viability or investment potential in adulthood. For example, females exposed to early life stress stemming from prevalent diseases should have placed greater value on – and may have had higher thresholds of acceptability for – a mate's degree of viability. Conversely, women should have placed more weight on the willingness and ability of mates to invest if the primary source of early stress was poor or unpredictable food supplies. The important point is that the specific source of psychosocial stress in a person's past could influence her or his tendency to value, attend to, and hold higher or lower standards of acceptability for a potential mate's viability in relation to investment potential, or vice versa. These standards may also have influenced decisions about whether and when to terminate relationships.
Though taken for granted in modern environments, our ancestors faced major obstacles in raising even a few offspring to adult reproductive age. Selection pressures should have led people to generally make adaptive trade-offs on the basis of reproductively relevant events that occurred earlier in their lives. However, selection pressures should also have led people to make adaptive trade-offs in response to reproductively relevant conditions in their current environments. Long-term fitness returns might actually have been more strongly linked to the mating and parenting trade-offs that individuals made in response to their current environments in adulthood than to more distant life-history factors, especially if factors that affected mating or parenting changed within a person's lifetime (e.g., the prevalence of disease, sex-ratios, changes in the food supply).
