Starting at the beginning, biologists still face unanswered questions about the biological processes during gestation that determine the sex of a child. While the biological concept of "sex" -- referring to anatomical and genetic factors that make up an organism's sex -- is not equivalent to the social concept of "gender" -- a concept that includes behaviors and aptitudes typically attributed to men or women -- research is focused precisely on identifying some of the genetic and anatomical characteristics of an organism's sex that may explain gender attributes. As a result, biologists are investigating the genetic makeup of the Y chromosome, a chromosome found only in males, and comparing it against the genetic makeup of female DNA containing only X chromosomes. Specifically, biologists are interested in what factors lead to the creation of Y chromosomes (making a fetus male) and what genetic material is unique to the chromosome, hoping that some of these processes and materials may also serve to explain differences in gender behavior.
Biologists recognize that significant differences exist in hormonal functioning between the sexes. Even in the case of hormones like adrenaline found in significant amounts in both sexes, the processes of hormone production and distribution are somewhat different across the sexes. Considering the impact of hormone production on human behavior and even anatomy, biologists study these patterns, guided by questions like "Does hormone production respond to different stimuli across different sexes?" to tie clinical biological data directly to behavioral observations linked to gender roles.
As biologists, and neurobiologists in particular, have focused on the brain as the command-center that determines human behavior, a good deal of biological research on sex and gender differences has focused on brain functioning. Research areas include physical studies of the brain to find differences and research into the functioning of neural activity to measure different neural responses to similar situations. On the more applied side of biological research, scientists have attempted to document, often with controversial results, differences in spatial understanding and fine motor skills across the sexes to explain social trends like female roles in the fine arts and detailed assembly or male domination of fields such as engineering.
As part of the study of the development of sex differentiation in humans, biologists often focus their studies on individuals who do not fit into traditional sex and gender categories, such as transgendered or transsexual individuals. As the terms suggest, these individuals exhibit traits of both male and female gender and sex categories, either by having mixed anatomical features (transsexual) or exhibiting behaviors that transcend traditional boundaries (transgendered). These studies produce a number of results that challenge the traditional dichotomous relationship between male and female, also challenging the traditional correspondence between biological sex and social gender identity and shedding some light on the role that biological factors play in the construction of gender behavior.