Sexual selection is governed by social selection, and social regulation should also be taken into account. The “helper” hypothesis raised by Del Giudice is exactly this kind of mechanism. Del Giudice's characterization of the stress-stricken male as being more aggressive, more dominant, and more competitive may be considered as the early selection of boys to raise the cutoff point of survival, leaving only the highly fit individuals for adult stages when same-sex competition determines the odds for reproduction. This is buttressed by a series of studies in the obstetric literature from this decade, indicating the greater vulnerability of male fetuses as compared to female fetuses (Anderson & Doyle Reference Anderson and Doyle2008; Deulofeut et al. Reference Deulofeut, Dudell and Sola2007; Ingemarsson Reference Ingemarsson2003; Jones et al. Reference Jones, Karuri, Cronin, Ohlsson, Peliowski, Synnes and Lee2005; Pressler & Hepworth Reference Pressler and Hepworth2002).
Evolutionary logic may also explain the observed phenomenon. Ingemarson (2003) offers a view of the fetus as basically female. The masculinization process is regarded as an excess process that brings an additional set of risks to the fetuses' development. This could be interpreted similarly to the evolutionary Handicap Principal, according to which, excessive survival risks characterizing animals convey a social message of fitness over obstacles (Cellerino & Jannini Reference Cellerino and Jannini2005).
It may be argued that males have a genetic disadvantage because they are equipped with two distinct sex determination chromosomes (XY) instead of the parity backup that exists in females, who have two of the same kind of sex determination chromosomes (XX). Ingemarsson (Reference Ingemarsson2003) claims that in comparison with the X chromosome, the Y chromosome has degenerated through evolution and includes only a small number of genes, all of which are heterozygote. Having only one copy of each gene means that every “bad” allele is expressed.
A mediating factor for the gender genetic differences may be stress (Als Reference Als1986; Als et al. Reference Als, Lawhon, Duffy, McAnulty, Gibes-Grossman and Blickman1994; Reference Als, Duffy, McAnulty, Rivkin, Vajapeyam, Mulkern, Warfield, Huppi, Butler, Conneman, Fischer and Eichenwald2004; Heckman et al. Reference Heckmann, Hartmann, Kampschulte, Gack, Bodeker, Gorther and Wudy2005). Phillips (Reference Phillips2007) claims that antenatal stress has life-long effects that vary among men and women, and suggests the possibility of gender dimorphic environmental fetal programming. Davis and Emory (Reference Davis and Emory1995) show gender dimorphic stress reaction in healthy, full-term infants prior to extensive socialization.
Therefore, I suggest that these differences found in the response to stress are constructed and programmed in utero and continue to develop differently during the socialization process, so that the factor of stress, as suggested by Del Giudice, only secondarily affects gender differences, interacting after birth with this early programming. Meaney and Szyf (Reference Meaney and Szyf2005) examined this issue with an animal model and found that increased levels of pup licking/grooming by rat mothers in the first week of life alter the DNA structure at a glucocorticoid receptor gene promoter in the hippocampus of the offspring. These effects are far more robust in females, suggesting that girls may be better equipped genetically but may also have a greater vulnerability to non-supportive environmental signals, and that this is the manner in which the non-secure girl develops an anxious attachment style, as claimed by Del Giudice.
Findings from Francis et al.'s (Reference Francis, Young, Meaney and Insel2002) animal studies are in accordance with the above. Francis et al. found that in the central nucleus of the amygdala and bed nucleus of the stria terminalis (which encompasses discrete, anatomically separate populations of oxytocin and vasopressin receptors; Huber et al. Reference Huber, Veinante and Stoop2005), oxytocine receptors binding – which decreases anxiety and stress, and facilitates social encounters, maternal care, and the extinction of conditioned avoidance behavior – was increased in adult females, but not adult males, that had received high levels of maternal licking and grooming as pups. Conversely, amygdala vasopressin receptor binding – which is involved in modulating fear and aggression – was increased in males, but not females, that had received high levels of maternal licking and grooming. This further explains the manner in which the non-secure boy, in order to appear robust in the environment, develops – as suggested by Del Giudice – an avoidant attachment style.
The attachment styles may be understood not as gender differences so much as differences in the gender construct of society and its interaction with the genetic fetal equipment for dealing with intra- and extra-uterine stress, leading female offspring to be better fitted to deal with social pressure by seeking bonding and male offspring to be better fitted to deal with social pressure by avoiding collision. Therefore, finally, the gender effect could be explained by socialization: It could be that mothers care for boys and girls differently, according to certain social gender roles, as well as certain cues coming from the infant which are already formatted in utero.
The social context of my assumptions suggests that the interplay of sex hormones and stresses is an interfering factor in the aromatization process of masculinization, whereas Del Giudice refers to sex hormones as mitigating stress. In the face of contradicting results and theories, the concept of co-regulation may be considered (Als Reference Als1986; Goldstein Ferber Reference Goldstein Ferber2008; Hofer Reference Hofer1994). That is, in situations of good co-regulation between neurobehavioral subsystems in utero and in the newborn period, a boy may benefit from the interplay between these subsystems, including the development of the sex hormones; whereas, in cases where such a co-regulation state is lacking, especially in the deregulation of the development of the hypothalamic-pituitary-adrenal (HPA) responses, the interplay may turn into an interfering process and result in difficulties as early as during gestation.
Additional perspectives suggest that within a regulatory process in early parental interactions, with the offspring's cues already shaped in utero, the dyadic reciprocal regulation between the child and his or her parents in these early interactions (Archer Reference Archer1996; Cho et al. Reference Cho, Holditch-Davis and Belyea2007) may determine (1) whether the boy or the girl develops a secure attachment style, and (2) whether the boy or the girl develops his or her gender's typical disadvantage in attachment style (i.e., either avoidant or anxious).
Therefore, I suggest that the level of fitness between the gender-type cues shaped during gestation, the ability of the parents to reciprocate with those cues, and the social interest the parent represents, work in feedback circuits. Having said that, it seems that socialization and the development of attachment styles are processes provided with windows of opportunities during critical ages in early childhood, and that anthromorphologic structural theories are able to explain contradicting research data by functional ante- and postnatal processes.
