Brains, Genes, Nature and Nurture

I'm still cogitating over my discussions with Dr Barbara David the other day. Several points she made have stuck in my mind.

The first is that correlation does not imply causality in any particular direction. Thus if two observations A and B are correlated, it is possible that A causes B, B causes A, or that both A and B are causes by a (possibly unobserved) C.

When it comes to TS, the BSTc layer difference between males (including transsexual males) and females (including transsexual females) is strongly correlated with self-perceived gender identity. The experiments and autopsies involved small numbers (n~=12), but covered virtually the entire population - all TS women who had died in the Netherlands during the experimental period. The sample should not be seen so much as n=12, but n=U, the universal set, everybody. Or rather, since we're talking autopsies, every Body.

The point is though that the BSTc layer differences appear after gender identity is formed. They appear around Puberty, while gender is set somewhere between ages 2-10 in almost every case. Not quite always, but the exceptions are outliers. Often gender is only perceived when compared with others: a child only "knows" their gender when they socialise with other children, who are gendered one way or the other. But it's set beforehand. I might add, this corresponds exactly with my own experience.

So gender identity is not caused by this neural difference: the neurological difference is symptomatic, and appears to be reliably symptomatic, but is not causal.

The second point she made (as I understand it) is that the brain's neural network is essentially unformed at birth. The neurons are there, in situ, but the interconnections, the things that make a brain a brain, are mostly not. A bag of silicon chips is not a brain, a set of pigments and brushes is not a painting. Moreover, the brain is plastic: environment shapes the way the brain is formed. This is obvious in the rare and tragic cases of children who have been brought up without human contact, such as feral children adopted by animals, or more commonly, victims of extreme child abuse who have spent much of their formative years in "solitary confinement", locked in cupboards. Their brains do not develop normally, many cannot learn to speak, those parts have developed in different ways and cannot easily be changed once set.

So the brain is plastic, but loses plasticity as we grow older.

The question then arises, what is the cause of Transsexuality? Is it due to experience after birth during the first dew years of life? We know that female babies behave differently from male babies, but they also get treated differently. Girls get hugged closer than boys, for example. Could it be that environment is causal? Girls are girls because they are treated as girls while neonatal?

The difference in biology extends to the 3rd week after conception. From Scientific American :

...one thing that doctors know for certain is that in the second and third trimesters, women having girls display higher levels of a hormone known as maternal serum HCG (MSHCG) than do women pregnant with boys. Now new research suggests that such hormonal differences appear less than three weeks after conception. The findings, published today in the journal Human Reproduction, may help explain how girls and boys exert control over their mother's hormones.

Yuval Yaron of the Genetic Institute at Sourasky Medical Center in Tel Aviv and colleagues followed 347 pregnancies achieved through in vitro fertilization. The researchers tested the mothers' MSHCG levels between 14 and 20 days after fertilization and detected some differences as early as day 16. Three weeks into pregnancy, women carrying girls exhibited hormone levels 18.5 percent higher than those of their boy-carrying counterparts, regardless of factors such as previous pregnancies or maternal age.

It seems entirely, er, conceivable, that limitations and constraints that will bound the development after birth may be set very early, at the beginning of development as a vertebrate.

So is Nature or Nurture dominant in neural development? There's proof that in some areas, Nature certainly sets bounds. From BrightSurf.com. October 17 this year:

While showing an impressive growth prenatally, the human brain is not completed at birth. There is considerable brain growth during childhood with dynamic changes taking place in the human brain throughout life, probably for adaptation to our environments.

Evidence is accumulating that brain structure is under considerable genetic influence [Peper et al., 2007]. Puberty, the transitional phase from childhood into adulthood, involves changes in brain morphology that may be essential to optimal adult functioning. Around the onset of puberty gray matter volume starts to decrease, while white matter volume is still increasing [Giedd et al., 1999].

Recent findings have shown, that variation in total gray and white matter volume of the adult human brain is primarily (70-90%) genetically determined [Baare et al, 2001] and in a recent magnetic resonance imaging (MRI) brain study with 45 monozygotic and 61 dizygotic 9-year-old twin-pairs, and their 87 full siblings also high heritabilities have been found [Peper et al, in preparation]. Thus, while environmental influences may play a role in later stages during puberty, around the onset of puberty brain volumes are already highly heritable.

So genetics play a strong part in shaping neural morphology. Or do they? Again, does A cause B, B cause A, or are A and B caused by C? It could be that the link is indirect, that genetics tend to cause a hormonal and biochemical environment which then causes constraints, tendencies and biases in further development.

That this is likely is evidenced by one fact: that 1 in 6 xy-foetusses whose mothers were given DES in the first trimester develop as transsexual women, not men. The causation is not direct, and things can go awry. DES causes other developmental anomalies too. Furthermore, there is no actual evidence that TS people have any difference in their upbringing (in the first 2 years and in pre-natal stimulus) than others who are not TS. They may do, but if so, we have no evidence of it.

Overall, I, from my mechanistically-biased view of cognition, tend to favour inherent biological causes, while Dr Davids, from her long experience as an expert psychologist, is perhaps more aware of the possibilities and plasticity in human development.

I still think I'm right though, even taking that into account. Too bad I can't do simultaneous PhDs in Gender Studies, Psychology, and Computer Science. Life's too short though.