Sample sizes

Whenever it's relevant (when feminism comes up, or materialism) I talk to my students about the studies of sex differences in the brain.  The press always reports any difference discovered by researchers as proof of innate differences in cognition.  Of course, the biggest problem with that interpretation is that environmental factors affect the brain, so finding something in the brain is not useful for determining where it came from, but I also mention the small sample sizes the studies usually have (a couple of dozen participants at most).

A friend is getting his Ph.D. in neuroscience (incidentally, according to him MRI studies, which are the kind that usually get such press, are all crap) decided to practice his thesis defense presentation on his friends before doing the real thing, so I heard about his research.  It had nothing to do with sex differences (it was about some details of how motor control works in the brain), but I learned something about sample sizes.  I was somewhat shocked to discover that it involved two experiments with a grand total of three monkeys (two per experiment; one monkey was involved in both).  I guess these were hideously expensive cyber-monkeys (they had electronic implants to measure brain activity very precisely during the experiments), so it wasn't practical to use more, but it seems like one could get very deceptive results this way if one were unlucky and got even one atypical monkey.

My skepticism of overly confident neuroscience claims has been further increased.

Comments

[Andy McKenzie]

You may find this recent J. Neuro article interesting:

http://www.jneurosci.org/cgi/content/abstract/29/45/14265

Here's the main part (IMO) of the conclusion:

"Regional GM is a composite measure of different microunits, such as neuronal bodies, axons, dendrites, synapses, glia cells, and others. Given that MRI signal strength is somewhat related to cellular characteristics, it is tempting to immediately relate the larger female GM volumes to specific cognitive functions (i.e., in which women show better performance). For example, if a larger regional GM volume reflects more numerous neurons, such tissue enlargements might be advantageous by facilitating an efficient processing of ingoing and outgoing information, which might be beneficial for cognitive performances. In support of this assumption, histological data has shown regionally increased female neuronal densities in the posterior temporal cortex and greater dendritic values in Wernicke's area... one could speculate that the observed larger GM volumes in females are associated with women's superior language skills."