02 Jan 2020
Future research is likely to be aided by improved methods of visualizing the living human brain. Research in nonhuman mammals reported sex differences in cell groups, often of only a few hundred cells, that cannot be visualized in the living brain using current technologies. The small magnitudes of sex differences reported to date in the living human brain may partly reflect an inability to look more closely, particularly at small subcortical cell groups. In addition, thus far, human research has looked largely at relatively gross characteristics, such as regional volumes and fiber tracts. Animal research, however, has found that many more subtle characteristics, such as neurochemical phenotypes, dendritic branching, and synaptic densities, to name just a few, show sex differences (
McCarthy et al., 2015). Analyzing these types of characteristics may increase the explanatory power of human research. In addition, researchers developing animal models and those studying humans have not always communicated as well as they might have. It would be interesting to know, for instance, which of the small sex differences that have been seen consistently in the living human brain are also seen consistently in other species, and whether these differences in other species are influenced by early androgen exposure. Similarly, researchers examining early hormone effects on human brain and behavior might benefit from closer attention to the specific hypotheses suggested by the large body of relevant research in other species.
Recent work has benefitted from attention to issues of reliability, sample size, and statistical procedures. Additional attention is needed to sampling biases as well. More information on sex differences over the life span, beginning at birth, would also be useful. Early life is a time when interventions may have maximal impact. Future research might also explore how different types of factors, such as early testosterone exposure and parental socialization, work together in the developmental system that produces sex/gender differences in brain and behavior. Measures of testosterone and parental behaviors during the first few months of infancy (mini-puberty) might be useful in pursuing this goal.