Tuesday, May 24, 2005

Does nature or nurture discriminate against men?

In my last posts, I have spoken about the sex ratio in PDS. About 80% are men and 66% 3-year old boys who stutter. Why??? What is the reason that the figure is not 50%-50%? Is it nature or nurture that discriminates against men/boys??

The nature/nurture discussion has always been a very heated one, especially as scientific evidence regularly clashes with cherished political positions and public attitudes (from the left and right political spectrum!) e.g. on gender/race differences, or causes of criminality/murder/rape. When reading some of the PDS literature, I have the feeling that often science comes second to sticking to cherished views. Anyone interested in these debates, I recommend reading Steven Pinker's book "The Blank Slate". But also his earlier books "How the Mind Works", and "The Language Instinct". His books are just absolutely outstanding in quality and clarity. In "How The Mind Works", he gives an update on what science has found out about human nature, and weaves it into a cohesive picture of human nature. I have found it a very useful mental tool to get a grip on the nature of PDS on the various level and put it into a cohesive picture.

On the sex ratio, here are some arguments to consider:

1) By asking "Is it nature or nurture?" I have committed on purpose a logical fallacy for the sake of attention seeking. Rarely is a human phenomenon only nature or nurture. Some phenomena are clearly nurture like which language you speak or which religion you practice. Others are nature like hereditary disorders. But mostly it is a mixture of both, of the genes and the environment.

2) The gap between the genders is increasing with time from about 2:1 for 3 year olds to about 4:1 for adults. But quickly stabilize lets say after age 7.

3) This sex ratio discrepancy is across virtually all developmental disorders like autism, learning difficulties, speech and language disorder in general, and more. As Patricia Sims has pointed out in her recent comment . I don't know of any article that discusses this. But ultimately, a theory must come up with an explanation of why these ratios exist and why they are different across disorders.

Recent research has shown further difference between women and men on the genetic level in addition to sex hormone differences: Apart from men having a Y chromosome, women (who have two X chromosomes with one being active) are able to activate the inactive one to some degree. The authors speculate that "these differences should be recognized as potential factors for explaining normal differences between the sexes but also gender differences in how certain diseases are manifested, progress and respond to treatment." In general, the female brain is also more plastic (e.g. men recover less well after strokes) and greater interaction between left/right brain (have a thicker corpus callosum, evidence from brain imaging during mental tasks), and also develops faster (eg girls start speaking earlier). All this would fit well the evolutionary arguments that it is far more important for the survival of a tribe that nature ensures stable and quick development of fertile women than of men as women have limited reproductivity and men no limits.

4) The difference between genders is not just a few percent, but four times are as many men. This is a very clear difference.

5) A recent brain imaging studies by Ingham et al. has shown some gender differences in brain activation.

My view on the origin is very similar to the one given by Buechler and Sommer: "Given that boys are about four times less likely to recover from stuttering than girls, it is tempting to speculate that all stutterers have a slight abnormality, but only those that can use the right hemisphere for language can develop into fluent speakers. Language lateralization is less pronounced in women (McGlone 1980) and might therefore be related to the fact that women show an overall lower incidence in PDS." Girls probably have a higher recovery rate or may have better compensation techniques. So I would say that nature has done the discrimination to a very large degree. The environment might only be relevant in DECIDING WHO of the disfluent kids might recover and who doesnt. In a sense Nature says to Environment: Take care of these kids, girls will recover more often. I dont care who you choose. Environment it is up to you to decide but keep the sex ratio.



Anonymous said...

Tom, the findings of Drayna et al (Drayna, D., Kilshaw, J., & Kelly, J. (1999). The sex ratio in familial persistent stuttering. American Journal of Human Genetics, 65, 1473-1475) add another wrinkle to the issue you are discussing. This finding of a suggested familial history link to the sex ratio needs to be replicated, but it adds something more to equation.

Anonymous said...

Tom, what you say about stammering, tourettes and autism ratios between male and females is interesting. There are also other ways in which males are different from women and these manifest themselves in strange and unusual ways. For example males tend to have superior mathematical and visual spatial abilities, men seem to be better at chess (1 top female gramdmaster out of 200-300 around the world). Males are more likley to get a 1st class honour at a top University. Men are also more likely to be drawn to behaviours from coin and stamp collecting to train spotting to world changing inventions and creativity (ie Einstein, Newton, Darwin, Di Vinci etc). Bsaically, my point is that as men we are more likely to get someting wrong with us, but also more likely to excel in some other area. Maybe nature is equitable after all.