A Trio of Genetic Studies Relevant to Transgender Causation

Lately, I have had some time to catch up on my backlog of scientific papers that I have accumulated but have not had the time to read. I found some interesting genetic studies which may seem obscure at first but actually have direct implications for transgender folks. So here are my latest finds:

Finger Ratios Are Under Genetic Influence

You hear about the prenatal testosterone theory of transgender (PTTT) causation all the time. It appears in numerous scientific papers as well as in the popular press. Essentially the theory originated by Gunter Dörner in East Germany about 50 years ago says that being transgender is caused by abnormal levels of testosterone in the brain during development in the womb. So the story goes, trans women are exposed to lower testosterone levels and trans men are exposed to higher levels in the prenatal environment.

I have previously written about the evidence that disconfirms the PTTT theory but I recently ran across a paper provides additional evidence (Gobrogge, Breedlove & Klump, 2008). Breedlove has an excellent reputation for his really good work which indicated that male and female brains are anatomically different. But, in this case, he was looking at genetic differences in gross anatomy between the sexes, the 2D:4D finger length ratio. This ratio is often cited as a surrogate for prenatal testosterone levels during critical growth periods that determine behavior predisposition but there is little convincing evidence of this. It is beyond the state-of-the art to directly measure testosterone levels during the prenatal period.

Of particular interest to us is that 2D:4D finger length ratios (index finger to ring finger) have been found to correlate with being transgender (higher ratios in trans women). I have maintained that this ratio is under the control of genetics, not prenatal testosterone levels because it varies in people from region to region in the world. From other evidence, we know that finger length is controlled by a particular genetic network (HOX) and that finger ratios can be selectively bred in animals.

The study that I unearthed from 2008 supports the genetic control of 2D:4D ratio using the conventional test of genetics in humans—twin studies. Gobrogge compared the ratios between identical twins with the ratios between fraternal twins. The results indicated that 2D:4D ratio is under genetic influence and not environmental influence. Identical twins have exactly the same genes at conception whereas fraternal twins that do not have the same genetic makeup but do share most of the prenatal environment. The correlation between identical twins was high for genetic factors and non-shared environment but not for shared environment.

I am still waiting for a study that looks across the HOX network for genes which are part of the gender predisposition network. Thiessen (2019) found 400 or so genes that correlate with being transgender but have not released their full results. Are any of these in the HOX network? Are you listening down there in Augusta, Georgia?

Pretty soon, I am going to dramatize in fiction the PTTT as one of several transgender horror stories.

Genetic Markers for Handedness

Genetic analysis and MRI are getting cheaper and I found a scientific article which gives us better evidence of the genetic contribution to handedness and how it influences the growth of the brain. Why should transgender people care about this? Turns out that there are several studies which indicate that transgender people are less right handed than the general population. Most people are not totally left or right handed—meaning that they do everything with one hand. Handedness refers to how many typical tasks one tends to perform with left or right hand. For example, I write with my right hand but swing a golf club with my left. And there actually are two scales which assesses handedness in a subset of tasks. Transgender people tend to do more tasks with their left hand than their right compared with controls.

So what causes handedness to vary, anyway? There have been several studies implicating genetics as a factor. For example, parents who are both left-handed tend to have a higher percentage of children who are also left-handed. But in an elegant study by Wiberg, et. al. (2019) researchers at Oxford tied handedness to both genes and brain anatomy assessed by MRI. Starting with an existing database of over 400,000 people in the U.K. (U.K. Biobank) that included asking people whether they were left- or right-handed. This database also included full genome scans and MRI brain scans.

The study results indicated that there was a network of genes with four focal points on the 1, 2, 17 and 22 chromosomes which correlated with handedness and as a result that left-handers had more white brain matter connections indicating better communication between the left and right halves of the brain. The left-handers have better connections between functions which are performed in both halves of the brain. Language is usually performed in the left hemisphere but there is still some representation in the right hemisphere for this function.

This is also consistent with MRI studies which indicate that transgender people have more white matter in the brain than other people. White matter is white because the parts of the neuron that communicate with other cells are coated with fat and the fat speeds up transmission from one part of the brain to another.

So, the lefties are better able to communicate between, say the left brain language centers with the right brain language centers. The additional connections are mediated by identified genes that create microtubules, hollow cylindrical structures that guide the neurons as they make connections in the early brain. So now we can relate genes to anatomical structures in the brain and nervous system. This model may also apply to genes involved in transgender predisposition. And as with the 2D:4D finger length ratio case, we need to compare the transgender predisposition genes with the handedness genes.

Only problem with this study is that a handedness scale was not used; people were just asked if they were left or right-handed which means that statistical power was lost. But the authors did not craft that question in government surveys and had to make the best of what they had.

Female Big Game Hunters

The study of hunter-gatherer societies is important for transgender people because the Traditional Theory of gender evolution postulates in hunter-gatherer tribes males left home base to do the hunting and females stayed home to take care of pregnancy and childcare. So, the story goes, males and females inherited gender behaviors due to this division of labor. Later evidence suggested that the females did stay close to home base, due to pregnancy and nursing but they still accounted for the majority of calories produced by the tribe. Females gathered food and did some small game hunting near home. When the area around a home base was depleted of gather-able calories or other resources, the tribe simply moved on to another home base. Females also participated in war to defend the tribe. The Corded Ware peoples (3100 BCE-2350 BCE) of the U.K. were known to bury females with full battle dress in respect. And 75 tribes of the U.S. Plains Indians had a fourth gender who were females who concentrated on warfare skills. And in recorded history there are numerous cases of female warriors such as Boudica in present day U.K.

But accumulated evidence from early burial sites in the Americas (Haas, 2020) indicates that 30-50% of females also participated in big game hunting. The evidence involves 27 individual burial sites from approximately 9000 years ago which contain stone tools typically used for big game hunting. The tradition of these early cultures was to bury each person with their tools. Of those sites, 11 were determined to be female and 16 were determined to be male by dental and bone artifacts. It stands to reason that tribes living at the subsistence level needed maximum behavioral flexibility which included females also being skilled in the hunt for big game.

As the lead author of the study describes it, “Archeological discovery and analysis of the early burial practices overturns the long-held ‘man-the-hunter’ hypothesis”. “It’s now clear that sexual division of labor was fundamentally different—like more equitable—in our species’ deep hunter-gatherer past.” (Science Daily, 4 November, 2020).

This evidence tends to support the Kinship Theory of gender behavior evolution rather than the Traditional theory. The Kinship Theory says that the basis for evolution was the tribe. Those tribes with redundant, flexible behavioral capabilities would have increased evolutionary fitness and would be more likely to survive and pass on their shared genes to offspring. Behaviors, such as big game hunting, which would have been rigidly sexed under the Traditional Theory might be disassociated from sex under the Kinship Theory because such arrangements provided greater flexibility.

Conclusion

The first two studies above support the idea that a genetic predisposition, is formed in the brain such that when a child observes or is taught gender behaviors in early learning that they have emotional reactions that shape their subsequent gender behavior. The third study supports the idea that these gender behavior predispositions were not determined by sex but instead by the survival advantages of having diverse traits in each hunter gatherer society providing greater flexibility.

The three studies tend to support my previous analysis and conjectures about transgender causation in the Four Factor Theory. A theory is worthless unless it can be disconfirmed by evidence. I keep looking for data that disconfirms my theory, and will be sure to report them to you. It is short-sighted to cherry-pick results because negative results provide new understanding and result in improved, revised theories.

Tags: featured, science, Transgender

Category: Transgender Body & Soul