Sexing the baby: Part 1 – What do we really know about sex differentiation in the first three years of life?

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Abstract

The most common paradigm used to analyze health differences between men and women, is to divide the body from the social environment. In such a model, the bodily contribution and the social contribution add up to 100%. A few health science researchers offer more sophisticated approaches. None, however, offer an intensive study of the first several years of life in order to offer a model which integrates biology and culture in a fashion that demonstrates the productive processes by which gender itself emerges. In this article, we identify the earliest known sex-related biological and behavioral differences in young infants, toddlers and their parents and indicate how these might relate to health and disease. We frame these differences using unifying concepts from the study of neuroplasticity and dynamic systems theory.

Highlights

► We find early sex-related differences in toy preference and vocalization. ► Large individual differences at birth slowly take on a gendered valence. ► We describe current knowledge about sex-related differentiation at birth. ► Developmental dynamics is simultaneously a biological AND a social process.

Introduction

Men’s and women’s health problems and their health care needs sometimes differ. Similarly boys and girls face somewhat different developmental challenges. Many differences stem, self-evidently, from different reproductive anatomies and physiologies. More surprising have been non-reproductive differences, for example differing manifestations and timing of heart disease, differing rates of immune disease and varying patterns of mental illness (Institute of Medicine, 2001). Indeed, such sex-related variation presents a theoretical challenge to the health sciences. On the one hand it seems to call into question the idea that there is a universal physiology that can be understood via a universal norm. On the other, it seems to suggest an unlikely conclusion — that there are two possibly non-overlapping normals, one for women and one for men. Either model has important implications for health maintenance and disease prevention and treatment.

How then might we theorize the relationships between sex, gender, human physiology and health (both mental and physical)? The most common paradigm divides the body from the social environment, apportioning disparities between these two sources. In such a model, the bodily contribution and the social contribution always add up to 100% (Bierman, 2007, Society for Women’s Health Research,). In contrast, Bird and Rieker emphasize the view that biological differences can result from social causes (Bird & Rieker, 1999). Within the context of epidemiological health research, Krieger considers embodiment as a multi-leveled process, arguing that good theory and research practice ought to integrate body and psyche within specific social, historical and ecological contexts (Krieger, 2005). Her central claim, with which we quite agree, is that bodies offer us information about the conditions in which they grow and develop. A third mode of analysis—the Life Course Approach (Kuh and Hardy, 2002, Kuh et al., 1997)—has much in common with Krieger’s models and with the dynamic systems approaches we champion (Fausto-Sterling, 2000, Fausto-Sterling, 2003, Fausto-Sterling, 2005, Fausto-Sterling, 2008, Fausto-Sterling and García Coll, 2006, Fausto-Sterling et al., in press, Jimenez-Robbins et al., 2009, Sung et al., 2010).

To develop dynamic approaches to embodiment, including the body’s relationship to sex and to gendered social milieu, we need a starting point and a theory that can guide us as development proceeds. To this end we frame current knowledge about sex-related development in the first three years of life in terms of dynamic systems, an approach which emphasizes process-oriented, dynamic accounts of the body. Our framework integrates biology and culture in a fashion that has the potential to demonstrate the productive processes by which gender itself emerges and through which we can understand how seemingly sex-based differences in health are really due to the dynamic integration of biology and culture.

Researchers have produced a significant body of work on early-appearing sex-related behavioral differences (Hines and Collaer, 1993, Maccoby, 1998, Martin et al., 2002; D. N. Ruble & Martin, 1998; Diane N. Ruble, Martin, & Berenbaum, 2006). Some have parlayed these early differences into explanations of sex-related mental health differences, especially the differing rates of autism and ADD in boys and girls (Simon Baron-Cohen & Hammer, 1997; S. Baron-Cohen, Knickmeyer, & Belmonte, 2005). As with the study of health disparities, in the field of behavioral sex-related differences the predominant operating models – biological predisposition from neural and hormonal events during fetal development versus postnatal cognitive and social learning – have led to different types of experimentation and thus different and poorly connected bodies of knowledge. Although researchers on each side of the ontological divide acknowledge the importance of the other point of view, often relying on a poorly defined “interaction” term to link the two bodies of knowledge, such acknowledgment does not explain the emergence of sex-related differences in the early years. Here, we examine what we do and do not know about sex-related differences in biology and behavior from the prenatal period into the third postnatal year. Since researchers in this field often relate their findings to disparities in mental health, a careful analysis of the developmental baselines is crucial for critically assessing causes of mental health disparities. In leaning on perspectives adapted from dynamic systems theory we render visible, features of existing knowledge that we currently fail to notice and provide the study of gender and health with a grounding from which to develop better health-related knowledge (Spencer et al., 2006).

We present our findings in the following order: (1) what we know about behavioral differences between boys and girls in the first three years of life, (2) what we know about biological differences, (3) what evidence links biological differences to behavioral differences, (4) what we know about gender-related differences in behaviors from adult caregivers, and (5) the possible relationship of such behaviors to emergent sex-related differences in infants and toddlers. We limited our literature review to the prenatal period through age three, choosing the somewhat arbitrary cut-off point of 36 months because by that time there are several well-established sex-related differences. Our goal is to establish a time line for emergence of difference, which means starting before a phenomenon of interest is measurable in order to follow its appearance over time. Where possible we calculated effect sizes for reported differences using the online statistical calculators prepared by Lee A. Becker and available online at http://www.uccs.edu/∼lbecker/psy590/escalc3.htm as well as the computer program The Effect Size Generator (Devilly, 2004). Often the magnitude of differences starts out small in the face of large individual variability, but with age may become more established.

Most of the material we used to identify studies of sex differences in young children was obtained from searches of PsychINFO and Academic Search Premier. Initially we used the key terms human-sex-differences, and subsequently used a variety of other terms to narrow the search. We limited our review to articles appearing since 1950s. We also examined current child development textbooks to learn what were viewed as established sex differences and sought out the original works cited (or alluded to) in these texts. Several studies were also obtained from the reference lists of such fundamental works as The Psychology of Sex Differences (Maccoby & Jacklin, 1974), The Two Sexes (Maccoby, 1998) and Gender, Nature and Nurture (Lippa, 2002).

We excluded studies if they used only parent-reported measures of children’s behavior (except in measures of communicative behavior), reasoning that parents’ reports may be influenced by their expectations or assumptions (Seifer, Sameroff, Barrett, & Krafchuk, 1994). For the same methodological reasons, we would have liked to exclude studies in which the experimenters or observers are not blind to infant sex. However, blind studies are not a standard practice in this field. Yet this early work includes some of the most often cited sources and is critical to any review of the literature and we thus felt we could not exclude it. Many heavily cited studies had not been replicated. While we initially intended only to include studies that had been replicated twice or more, this so severely limited our data that we eventually chose to include even those studies with no replications. We included findings showing a main effect of subject sex with the relationship significant at the level of p < .05. When possible, we presented the means as given in the original work.

We also studied meta-analyses (e.g. Hyde and Linn, 1988, Leaper et al., 1998, Lytton and Romney, 1991) but did not include them in our data tables. Similarly, some studies revealed an interaction effect between sex and some other variable (e.g., birth order); while these are of interest, for the sake of clarity, we have not displayed them in our tables unless main effects of sex were reported. Furthermore, many reports present data on children at a range of ages (e.g. 2–5 year olds); unless means and/or statistics were given for children 36 months old and younger, we have not included these reports.

Section snippets

Activity levels

Recent metanalyses of infancy, have found a small but significant tendency for boys to be more active than girls (effect sizes ranging from .12 to .29) (D. W. Campbell and Eaton, 1999, Eaton and Enns, 1986). Reviewing studies of preschoolers (aged 12–72 months) Eaton and Enns (1986) found a moderate effect size (d = .44), suggesting that activity level differences between boys and girls might grow over time. We graphed the effect sizes versus age listed by Campbell & Eaton, and found that in

What physiological and anatomical differences need to be explained?

Biology has had its own “systems” revolution (Arnold, van Nas, & Lusis, 2009). In the future, knowledge of different levels of hormone production might extend to the examination of networks of gene activation, or the intersection of effects across varied levels of biological organization. Thus, the material reviewed in this section is ontologically “old-fashioned” and integrating knowledge about biology into accounts of behavioral sex differentiation will be the systems challenge for the future.

Prenatal hormones and activity levels

Although no direct evidence links prenatal hormones (of any type) to the types of sex-related differences in infant motor activity reported by Campbell and Eaton (1999), scattered reports suggest a general relationship between stress-related hormone levels and infant motor activity (D. W. Campbell & Eaton, 1999). For example, in a cohort of normal births, Rothenberg et al. (1996) reported a correlation between lower β-endorphin levels in the umbilical cord and decreased sensorimotor development

Adult perceptions

Reid reported that mothers rated their newborn sons as having broad, wide hands, noted that they looked tall, large and athletic, and appeared to be serious (Reid, 1994). In a similar study that also included fathers, Rubin, Provenzano and Luria interviewed 30 pairs of parents (15 with sons and 15 with daughters) within the first 24 h after birth (Rubin, Provenzano, & Luria, 1974). The newborn infants did not differ by sex for length, weight or Apgar scores, but parents described daughters as

Conclusion

On average, boys are slightly heavier and have slightly larger brains at birth. They have been exposed to higher levels of testosterone prenatally and experience a postnatal testosterone surge at three months, while girls have been exposed to higher levels of follicle stimulating hormone. Neonatal differences in motor activity level decline postnatally, but reemerge at 4 months. More complex behavioral differences, especially toy preferences and vocalization emerge toward the end of the first

Acknowledgments

This research was supported by the Ford Foundation, the Pembroke Center for Research on Women, the Salomon Fund, Wayland Collegium, and the Center for the Study of Human Development, all at Brown University. We are grateful to the participants of the Roundtable on the Emergence of Sex Differences for sharing their perspectives, and to the Ford Foundation and the Pembroke Center for funding the event. Special thanks to Daniela Corbetta, Lisa Serbin, Diane Poulin-Dubois and Eleanor Maccoby for

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