Home › Research › Zhou BSTc

Zhou Brain Research (1995): what the study does and does not show

"Studies show that trans women have female brains" — that claim often rests on one study: Zhou (1995) on the brain nucleus BSTc. It was a small post-mortem study of brains of people who had used hormones, from a single research group. According to the Cass Review, brain research has so far not reliably identified brain changes directly linked to gender incongruence.

What Zhou did

Zhou, Hofman, Gooren and Swaab (Nature, 1995) examined post-mortem brains of six male transsexuals (source) and measured the volume of the bed nucleus of the stria terminalis, central subdivision (BSTc). The volume was larger in men than in women, and in the transsexual participants the BSTc was female-sized (source). The six participants had used cross-sex hormones (source).

Limitation 1 — brains after hormone use

The authors themselves concluded that the size of the BSTc was not influenced by sex hormones in adulthood (source). The Cass Review, however, writes (para. 8.18) that most brain studies examined brains after treatment with feminising hormones, rather than before and after treatment, so changes could be caused by the treatment rather than by an inherent difference (source).

Limitation 2 — the difference only emerges in adulthood

Chung, De Vries and Swaab (2002) found that the difference in BSTc volume between men and women only becomes significant in adulthood, and call that late differentiation unexpected (source). That fits poorly with the idea that this brain feature is fixed early in development and would explain a sense of gender identity in childhood.

Limitation 3 — small sample, post-mortem, limited replication

Six participants is few for an anatomical claim. The Cass Review writes (para. 8.18) that most brain studies are based on small numbers and have not been reliably replicated (source). A follow-up study by Kruijver et al. (2000), with 42 participants, also found a female-range number of neurons in the BSTc in trans women (source), but it came from the same research group, and according to a critical analysis 26 of the 34 participants in its main analysis were the same as in Zhou (source). See also publication bias and scientific problems.

Limitation 4 — no reliable brain marker

The Cass Review writes (para. 8.20) that research in this area has so far not reliably identified brain changes directly linked to gender incongruence, and that even if it could, this would say nothing about cause and effect; brains change through plasticity (para. 8.17) (source). See also no brain marker and no measurable marker.

What this means

The claim that "trans brains" exist rests on small post-mortem studies from thirty years ago from a single research group. Cass also writes that it cannot be ruled out that a biological predisposition plays a part in some individuals, in ways not yet understood (para. 8.21 and 8.23) (source).