Sex category may be used as an independent variable in these analyses to decrease within-group variability, but if Sex category does not have an effect, this will only lead to a reduction of degrees of freedom. Similar findings have been reported following other forms of manipulation e. However, given that sex interacts with other factors to affect the brain, sex effects on the brain must be understood as context-dependent, where context relates to the specific environmental, developmental and genetic conditions under which sex effects were assessed and under which the animals developed. We found that regardless of sample, type of MRI, and method of analysis, substantial variability is much more prevalent than internal consistency. Moreover, this mosaic changes as we experience the world, and, as the animal studies reviewed above demonstrate, some of these changes may be sex-dependent [ 29 ]. Reporting actual data and effect sizes enables scientists to appreciate the extent to which sex category explains or does not the variability in their results [ 78 ].
This is because if human brains belonged to two distinct populations, then every study of brain structure and function, even if not designed to detect sex effects e. We recommend using both males and females as study subjects in any experimental design sex as a component of methods , because such inclusion is crucial for ensuring that we more fully understand species' variability than is possible when only males or only females are studied. Importantly, substantial variability is not a result of the overlap between females and males in each of the brain regions, as evident in Fig. The suggested practice is expected to decrease the overall number of animals in research, especially in domains in which animals are bred for a specific study as in studies involving genetic manipulations and the use of only one sex category leads to killing of animals of the other sex category. This is particularly timely in view of the NIH initiative to consider sex in every study [ 25 , 26 ]. The number of subjects in these datasets ranged from to How does this exercise apply to human brains, for which hundreds of regions have been analysed for sex differences in size, volume or other characteristics? Yet, when scientists and laypeople list differences between females and males in the brain, they often implicitly assume that the more differences there are, the more different are the two sex categories, ignoring the possibility that some—possibly many—differences may compensate for others for review and examples see [ 59 — 61 ]; see [ 62 ] for a detailed example of compensation in the dopaminergic system and [ 63 ] for similar considerations in the case of sex effects on gene networks. Therefore in laboratory animals, differences between the sex categories may indeed reveal the effects of sex rather than the effects of some chance difference between the sample of females and the sample of males in the study. A different sex-by-environment interaction determined the density of CB1 receptors in the ventral hippocampus, as the same manipulation three weeks of mild stress eliminated a sex difference in the density of these receptors in the ventral hippocampus. In laboratory animals, in which brain variability is restricted by the tight control of genetic, developmental and environmental conditions, sex category can be used as a variable to reveal sex effects. If this is so, then using sex category as a variable is both unnecessary and misleading. Results T1-weighted images of females and males Table 1 were preprocessed and assessed for gray matter volume using voxel-based morphometry VBM Fig. This is because if the effects of an independent variable such as Treatment, do not interact with Sex category, then even if there are sex differences for example, females are on average more resilient than males in all treatment conditions , this has no relevance for the study's question regarding the effectiveness of the treatment. Thus, even when considering only a single characteristic the density of CB1 receptors and only two environmental conditions no stress versus three weeks of mild stress , the hippocampus can take three forms: Analytical methods with the above characteristics have been developed for working with other types of data. With respect to using sex category as a variable in the analysis of the results, we will separate the discussion of humans and laboratory animals. The implications of the mosaic approach for future research We started our discussion with the assertion that the existence of differences between the brains of females and males does not allow us to conclude that brains belong to two types nor to characterize the relationships between sex and the brain. Such an alignment may be predicted by the classic view of sexual differentiation of the brain, according to which masculinization and defeminization of the brain are under the sole influence of testosterone 9. As a result, although such comparisons may well yield significant differences between females and males due to the high heterogeneity of the population , these differences would probably reflect a false-positive finding resulting from a chance difference between the two samples included in a specific study. Sex category may be used as an independent variable in these analyses to decrease within-group variability, but if Sex category does not have an effect, this will only lead to a reduction of degrees of freedom. Embracing the concept of the mosaic brain should lead to multivariate experimental designs in which possible sex category effects are examined under changing environmental and genetic conditions. In fact, she is just as likely in population 3 or more likely population 2 , to have a mosaic brain, i. Such characterization is necessary for studying sex effects on the brain as well as for studying brain structure, function and dysfunction in general. Indeed, direct assessment of brain function using functional imaging techniques most often reveals small, highly overlapping, group-level differences between human females and males. We hope future studies will fill in this gap. We believe that developing or adopting analytical methods that take into account the heterogeneity of the human brain is crucial for the advancement of human health and well-being.
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