A number of people have asked for links to actual studies, peer reviewed articles, and such. Here is a good starter set to introduce the current state of research:
Links for online articles:
Beyond XX and XY: The Extraordinary Complexity of Sex Determination
https://www.scientificamerican.com/article/beyond-xx-and-xy-the-extraordinary-complexity-of-sex-determination/?utm_source=facebook&utm_medium=social&utm_campaign=sa-editorial-social&utm_content=&utm_term= Funct Neurol. 2009 Jan-Mar;24(1):17-28.
Sexual differentiation of the human brain in relation to gender identity and sexual orientation.
Swaab DF1, Garcia-Falgueras A.
https://www.ncbi.nlm.nih.gov/pubmed/19403051"Are the Brains of Transgender People Different from Those of Cisgender People?"
"The Scientist", March 2018, Shawna Williams
https://www.the-scientist.com/?articles.view/articleNo/51914/title/Are-the-Brains-of-Transgender-People-Different-from-Those-of-Cisgender-People-/"Sex redefined", "Nature", 18 February 2015, Claire Ainsworth
https://www.nature.com/news/sex-redefined-1.16943#/spectrumThe September 2017 issue of Scientific American has a number of articles on gender and the brain.
Transsexual gene link identified
http://news.bbc.co.uk/1/hi/health/7689007.stm"Gender & Sexuality"
http://www.annefaustosterling.com/fields-of-inquiry/gender/Assorted Publications on the Topic...
G. Spizzirri et al, "Grey and white matter volumes either in treatment-naïve or hormone-treated transgender women: a voxel-based morphometry study", Scientific Reports Volume 8, Article number: 736 (2018)
A.-M. Bao, D.F. Swaab, "Sexual differentiation of the human brain: Relation to gender identity, sexual orientation and neuropsychiatric disorders," Front Neuroendocrin, 32:214-26, 2011.
J.-N. Zhou et al., "A sex difference in the human brain and its relation to transsexuality," Nature, 378:68-70, 1995.
F.P. Kruijver, "Male-to-female transsexuals have female neuron numbers in a limbic nucleus," J Clin Endocrinol Metab, 85:2034-41, 2000.
A. Garcia-Falgueras, D. Swaab, "A sex difference in the hypothalamic uncinate nucleus: relationship to gender identity," Brain, 131:3132-46, 2008.
S.M. Burke et al., "Male-typical visuospatial functioning in gynephilic girls with gender dysphoria—organizational and activational effects of testosterone," J Psychiatry Neurosci, 41:395-404, 2016.
G.S. Kranz et al., "White matter microstructure in transsexuals and controls investigated by diffusion tensor imaging," J Neurosci, 34:15466-75, 2014.
E. Hoekzema et al., "Regional volumes and spatial volumetric distribution of gray matter in the gender dysphoric brain," Psychoneuroendocrino, 55:59-71, 2015.
L. Zubiaurre-Elorza et al., "Cortical thickness in untreated transsexuals," Cereb Cortex, 23:2855-62, 2013.
A. Guillamon et al., "A review of the status of brain structure research in transsexualism," Arch Sex Behav, 45:1615-48, 2016.
J. Junger et al., "More than just two sexes: the neural correlates of voice gender perception in gender dysphoria," PLOS ONE, 9:e111672, 2014.
I. Savic, S. Arver, "Sex dimorphism of the brain in male-to-female transsexuals," Cereb Cortex, 21:2525-33, 2011.
J.D. Feusner et al., "Intrinsic network connectivity and own body perception in gender dysphoria," Brain Imaging Behav, 11:964-76, 2017.
E.S. Smith et al., "The transsexual brain—A review of findings on the neural basis of transsexualism," Neurosci Biobehav R, 59:251-66, 2015.