The human mind was shaped by natural selection. The mind we inherited from our ancestors was tested against alternative versions and won out against the competition because it was more successful in dealing with the challenges of hunter-gatherer life. Would the mind depicted by attachment theorists have passed this test? Would it have helped its owner survive and reproduce in the Paleolithic?
According to attachment theory, an infant develops expectations about the world and its people on the basis of the way his mother takes care of him. How would this be beneficial to him? Why should a child with an attentive mother expect other people – siblings, peers, strangers – to be equally attentive? And how would it profit the child of a negligent mother to go into the world with the expectation that no one will love him? Isn't the child's mind capacious enough to form separate “working models” for all the important people in his life, so that he can behave appropriately with each of them?
According to the “life history” elaboration of attachment theory, a girl who experiences a stressful environment in the first few years of life, or whose father was absent or insufficiently attentive during that period, is more likely to reach sexual maturity early and to hasten into sexual relationships with temporary partners. But didn't every child grow up in a stressful environment in the Paleolithic? Many parents died before their children were grown. Polygyny was probably universal, limited only by men's ability to support additional wives. As noted in the target article, paternal childcare is rare in polygynous societies.
According to life history theorists, the child's experiences with parents in infancy and early childhood influence her reproductive strategy 10 or 12 years later. But why should a girl's reproductive strategy be based on such inadequate data? Many things might change in the interim, so why not wait as long as possible to settle on a strategy? And why look only at her own parents? Birds are capable of making a decision about next year's nesting site on the basis of “public information” they collect by observing other nests in the area (Doligez et al. Reference Doligez, Danchin and Clobert2002). If a bird can do it, why not a child? Her family may be atypical, so the more data she collects the better.
Attachment theory underrates the child. There is good evidence that children construct separate working models for each of the important people in their lives, and that expectations developed in one relationship are not carried over to other relationships. Babies can be securely attached to one caregiver and insecurely attached to another. Infants of depressed mothers are subdued with their mothers but behave normally with other familiar caregivers. Children who are pushed around by older siblings at home are fully capable of taking a dominant role with peers (see Harris Reference Harris1998; Reference Harris2006).
Although behavior is highly sensitive to context, it is not ruled solely by context. Researchers observed children interacting with their parents and the same children interacting with their peers, and recorded instances of uncooperative, angry, or bossy behavior. The correlation between behavior with parents and behavior with peers was .19 (Dishion et al. Reference Dishion, Duncan, Eddy, Fagot and Fetrow1994). I attribute this correlation not to generalization or transfer but to heritable personality traits (e.g., disagreeableness) that the child takes along to every social context. Confirmation comes from behavioral genetic studies of other behaviors, such as shyness. Correlations between shy (or bold) behavior in two different contexts are almost entirely due to genetic influences on these behaviors. In contrast, environmental influences on shyness are context-specific (Saudino Reference Saudino1997).
The same holds true for consistency over time. The long-term stability of personality traits is primarily due to the heritable component of such traits (Caspi & Roberts Reference Caspi and Roberts2001). This finding has an important implication: Personality characteristics that appear early in development and persist into adulthood are largely genetic and not, as most developmental psychologists have assumed, due to the lingering effects of early experiences.
Though the role of genes in producing resemblances between siblings is now generally acknowledged, the fact that children also share half their genes with their biological parents – both parents – is widely ignored. For genetic reasons alone, people who are impulsive, quickly bored, easily angered, or prone to depression have a greater-than-average risk of producing offspring with similar disadvantages. A pretest for temperament in infancy is a hopelessly inadequate way to control for genetic influences on behaviors measured in infant, child, and adult attachment tests (Rowe Reference Rowe2000b).
Attachment theory is based almost exclusively on studies that examine one child per household; almost always, the child is the biological offspring of the mother. This method makes it impossible to disentangle environmental and genetic influences. Studies of twins, siblings, and adoptees – two per household – have shown that about half the variance in most psychological characteristics can be attributed to genetic influences, and most of the remainder to unknown “unique” influences (see Harris Reference Harris2006). The environment shared by children who grow up in the same household accounts for little or none of the variance. Physical maturation – age at menarche – follows the same pattern; although heritability is higher, most studies find little effect of shared environment (see Mendle et al. Reference Mendle, Turkheimer, D'Onofrio, Lynch, Emery, Slutske and Martin2006). Whether or not a father is present, and whether or not the home is stressful, are aspects of the shared environment.
As Nettle (Reference Nettle2006) has argued, most individual differences in mating behavior can be attributed to heritable personality characteristics. Differences in rate of maturation are also heritable, and the environmental factors that play a role may not be the ones discussed in the target article. For instance, the role of socioeconomic status (SES) is often overlooked. Girls in troubled households may mature more rapidly because, in developed nations, obesity is negatively correlated with SES: children in lower-SES homes and neighborhoods tend to be fatter (Shrewsbury & Wardle Reference Shrewsbury and Wardle2008). Because age at menarche depends on body weight, overweight girls mature faster (Frisch Reference Frisch1988). The link between body weight and rate of maturation can also explain why girls today are maturing faster than they used to (Cesario & Hughes Reference Cesario and Hughes2007). The increased prevalence of obesity is probably to blame.
In conclusion, correlations between infant attachment behaviors and later sexual relationships are most likely due to persisting, heritable personality traits. Evolutionary psychologists and biologists can explain sex differences in sexual relationships without recourse to attachment theory.
