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14 The Splendor of the Human Person: A Catholic Vision of the Person and Sexuality Many differences between the male and female sexes are a result of the differ- ences between the genetic makeup of male and female cells and the differences in the expression of the genetic makeup. Sex differences have been identified at many levels of biological organization, from biochemical to behavioral. 10 These sexual differences have been shown by numerous studies that have found critical differences, including: females have more aggressive immune systems than males; female and male cardiac issues are significantly different; male and female brains develop and "age" differently; and male and female brains are or- ganized differently for language, with men relying on the left inferior frontal ridge of the brain for language tasks, while women use both the left and right inferior ridges. PET scans and MRIs demonstrate many other differences between male and female brains. Professionals can identify a male or female brain by sight, both before and after birth. Brain imaging studies have shown that women have a high- er percentage of gray matter while men have a higher percentage of white matter. 11 These differences, along with bone structure, muscle mass, heart capacity, and overall flexibility, are but a few of the findings that confirm the unmistak- able biological differences that exist between males and females, all a result of their genetic makeup. Although there are complicated and rare cases of disorders in sexual develop- ment, including situations where a person is born with organs of both sexes or where the sex organs are not completely developed one way or another, medical science usually can determine the person's sex. 12 Medical or surgical interven- tions in cases of disorders of sexual development may be warranted to address pathology or restore healthy functioning to the person's body. In contrast, medical or surgical interventions on the healthy body of a person who seeks body modifications for reasons of "gender identity" cannot be morally justified. 10 T.M. Wizemann and M.L. Pardue, eds. "Exploring Biological Contributions to Human Health: Does Sex Matter?" Journal of Women's Health and Gender Based Medicine 5 (June 10, 2001): 433-39. 11 Madhura Ingalhalikar, Alex Smith, et al. "Sex Differences in the Structural Connectome of the Human Brain," Proceedings of the National Academy of Sciences 111, no. 2 (2014): 823-28. 12 See "FAQs," Person and Identity Project, www.personandidentity.com.

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