[starbeast]

I generally take the view that any time you are dealing with a biological system, whether it be a single cell or the global economy, should you find yourself in a quandary where you are wondering if something is caused by this thing or that thing (nature/nurture being the classic), the answer is nearly always both, plus a whole bunch of some other stuff you hadn't even begun to consider. Biology is horrendously complex and messy, and anything built on biology is even messier.