Editing Sexual differentiation (section)

==Brain differentiation==
{{Main|Neuroscience of sex differences}}

In many animals, differences in the exposure of a fetal [[brain]] to sex hormones are correlated with significant differences in brain structure and function, which correlate with adult reproductive behavior.<ref name=":1"/> The causes of differences between the sexes are only understood in some species. Fetal sex differences in human brains coupled with early differences in experience may be responsible for sex differences observed in children between 4 years old and adolescence.<ref name=":5">{{Cite journal|last1=Fausto-Sterling|first1=Anne|last2=Coll|first2=Cynthia Garcia|last3=Lamarre|first3=Meghan|date=2012-06-01|title=Sexing the baby: Part 1 – What do we really know about sex differentiation in the first three years of life?|url=http://www.sciencedirect.com/science/article/pii/S0277953611003558|journal=Social Science & Medicine|series=Gender and health: Relational, intersectional, and biosocial approaches|language=en|volume=74|issue=11|pages=1684–1692|doi=10.1016/j.socscimed.2011.05.051|pmid=21802808|issn=0277-9536|url-access=subscription}}</ref>

Many individual studies in humans and other primates have found statistically significant sex differences in specific brain structures; however, some studies have found no sex differences, and some meta-analyses have called into question the over-generalization that women's and men's brains function differently.<ref>{{Cite journal|last1=Bishop|first1=KATHERINE M.|last2=Wahlsten|first2=DOUGLAS|date=1997-01-01|title=Sex Differences in the Human Corpus Callosum: Myth or Reality?|url=http://www.sciencedirect.com/science/article/pii/S0149763496000498|journal=Neuroscience & Biobehavioral Reviews|language=en|volume=21|issue=5|pages=581–601|doi=10.1016/S0149-7634(96)00049-8|pmid=9353793|s2cid=9909395|issn=0149-7634}}</ref> Males and females statistically differ in some aspects of their brains, still there are areas of the brain which appear not to be sexually differentiated at all. Some scholars describe human brain variation not as two distinct categories, and not even a maleness-femaleness continuum, but as mosaics.<ref>{{Cite journal|last1=Joel|first1=Daphna|last2=Berman|first2=Zohar|last3=Tavor|first3=Ido|last4=Wexler|first4=Nadav|last5=Gaber|first5=Olga|last6=Stein|first6=Yaniv|last7=Shefi|first7=Nisan|last8=Pool|first8=Jared|last9=Urchs|first9=Sebastian|last10=Margulies|first10=Daniel S.|last11=Liem|first11=Franziskus|date=2015-11-30|title=Sex beyond the genitalia: The human brain mosaic|journal=Proceedings of the National Academy of Sciences|language=en|volume=112|issue=50|pages=15468–15473|doi=10.1073/pnas.1509654112|pmid=26621705|pmc=4687544|bibcode=2015PNAS..11215468J|issn=0027-8424|doi-access=free}}</ref>

In birds, hypotheses of male-female brain sex differences have been challenged by recent findings that differences between groups can be at least partially explained by the individual's dominance rank.<ref>{{Cite journal|last1=Voigt|first1=Cornelia|last2=Gahr|first2=Manfred|date=2011-06-08|title=Social Status Affects the Degree of Sex Difference in the Songbird Brain|journal=PLOS ONE|language=en|volume=6|issue=6|pages=e20723|doi=10.1371/journal.pone.0020723|issn=1932-6203|pmc=3110770|pmid=21687671|bibcode=2011PLoSO...620723V|doi-access=free}}</ref> Furthermore, the behavioral causes of brain sex differences have been enumerated in studies of sex differences between different mating systems. For example, males of a polygynous [[vole]] species with intrasexual male competition have better spatial learning and memory than the females of their species, but also better spatial learning and memory than all sexes of other closely related species that are monogamous; thus the brain differences commonly seen as "sex differences" have been instead linked to competition.<ref name=":4">{{Cite journal|last=Geary|first=David C.|date=2017|title=Evolutionary framework for identifying sex- and species-specific vulnerabilities in brain development and functions|journal=Journal of Neuroscience Research|language=en|volume=95|issue=1–2|pages=355–361|doi=10.1002/jnr.23794|pmid=27870407|issn=1097-4547|doi-access=free}}</ref> [[Sexual selection]] does play a role in some species, though, as males who display more song behaviors are selected for by females⁠—so some sex differences in bird song brain regions seem to have been evolutionarily selected for over time.<ref name=":4" />