Heya everybody,

I added a few new studies today that are very relevant. I remember when the first sex difference in the human brain related to transsexualism was found in 1995 (Zhou, Hofman, Gooren, Swaab), it was quite a revolution, and it caused a push-back – some people favouring a psychopathological explanation of transsexualism argued against the findings. I wonder what these people are thinking today, when practically every month gives us a new study on brain differences found in transsexuals. Today, the evidence for a biological cause that is connected to the formative phase of the brain before and after birth is persuasive.

• Kranz, Hahn et al. (The Journal of Neuroscience, 2014) looked at the brains of female-to-male and male-to-female transsexuals and compared them to healthy male and female controls. They found differences in the white matter structure between the four groups, with the brains of male-to-female transsexuals shifted from the male mean towards the female. For female-to-male transsexuals, it was the other way around: Their white matter structure was shifted from the female mean towards the male. The conclusion: “Our data harmonize with the hypothesis that fiber tract development is influenced by the hormonal environment during late prenatal and early postnatal brain development.”

• In a highly interesting study published in “Hormones and behaviour” in 2014 by Hamann, Stevens et al., brain responses from women with XY chromosomes and the CAIS disorder of sex development are measured via neuroimaging and compared to healthy men and women. CAIS stands for “complete androgen insensitivity syndrome”. The affected women have male sex chromosomes (XY), testes and produce male-typical levels of androgens – but they lack functional androgen receptors preventing responding to their male sex hormones. “Thus, they develop a female physical phenotype, are reared as girls, and develop into women.” When studying their brain responses to vieweing sexual images, it was found that “that men showed greater amygdala activation to sexual images than did either typical women or women with CAIS. Typical women and women with CAIS had highly similar patterns of brain activation, indicating that a Y chromosome is insufficient for male-typical human brain responses.” This is remarkable, as it shows that hormones are responsible for this particular gender difference, not sex chromosomes. Sex chromosomes are just one factor among others that influence gender. This study is summarised in an article in PsyPost from November 2014.

• Barrett, Redmon et al. find (Neurotoxicology, 2014) that exposure to prenatal stress is associated with masculinised play behaviour in girls and suggest that prenatal stress may have androgenic effects on female foetuses (and anti-androgenic effects on male foetuses).

• A study by Shumer, Roberts et al. in the “Journal of Autism and Developmental Disorders” in 2014 finds that autism in mothers is linked to gender queer behaviour in children.

• Auer, Fuss et al. write in “PLOS One” (2014) that change of sexual orientation after transitioning is frequent.

• Kahlenberg and Wrangham (Current Biology, 2010) find that “sex differences in chimpanzees' use of sticks as play objects resemble those of children”.

• Escudero, Robbins and Johnson publish a study saying that “sex-related preferences for toy real stimuli” may not be innate, but are caused by “maturational and social development that continues into adulthood”.

• Joel writes in “Biology of sex differences” in 2012 that human sex differences do not come in two isolated extremes, but are spread out in a continuum. The many observed sex differences are not consistent, i.e. male or female characteristics are arranged in a mosaic-like pattern in humans.

• VanderLaan, Blanchard (yes, that Blanchard, I’m afraid) et al. find (Developmental Psychobiology, 2014) that low birth weight seems to be linked to sexual orientation and possibly cross-gender behaviour and identity.

Phew, that’s it for now. More to come later! Peace and light ☺