I have heard that in the last 5000 to 10000 years human brains have been decreasing, about by 10% as of present date. My question is, how many skulls do we have in the date range to go by? Or how many skulls do we have that are 5000-10000 years old? Do we need many, or can we get a good estimate with just a few. Just alittle clarification on how we can determine our decreasing brain size would be helpful, thanks guys.

I believe there have been a few studies done that have yielded similar conclusions, but the most recent one I know about was done by a Chinese resaerch group who compared the average dimensions in a sample of 81 skulls (dated from 5500-6200 years ago) to a sample of modern humans (>500 skulls CT scanned). I'm not sure if the article is open access (=free for anyone to view) but at least the summary should be available to see here

http://onlinelibrary.wiley.com/enhanced … 76/#Survey

So statistically speaking, these sample sizes are big enough to yield relatively precise estimates of the difference in average dimensions between those samples. In making such comparisons more is always better of course. Note that in this case no samples from intermediate times were compared so we can't say for sure whether, for instance, the change has been at a continuous constant rate over the last 5000 years (at least not from this study alone).

Another very important point to make is that the size of intracranial cavity does NOT necessarily equate to have a more or less complex brain. Just because intracranial cavity size has decreased (if it has) says nothing about cognition or intelligence (however it is measured). Those abilities are largely related to the neuronal and synaptic density in the brain rather than total volume.

Surely brain size says _something_ about the complexity of the brain? Given that it's not the only factor, doesn't it play in to some degree?

true but the degree of folding and synaptic density are probably better correlates of "intelligence". the wikipedia link gives a summary of the issues I am referring to
http://en.wikipedia.org/wiki/Cetacean_intelligence