What explains the South-North gradient in German IQ?

n many countries one can observe a North-South gradient of IQ and height. The likely explanation is that being big and smart is more beneficial or rather more necessary in colder climate, and many animals show this pattern. In Germany however, there is a distinctive IQ gradient from South to North, while for height it is the other way around. Because of the Alpes in the South and the Sea in the North, the South has a harsher climate, which would serve to explain the IQ gradient. But why does the height gradient not follow suite?

Probably, because selection for IQ is much more recent than selection for height. Height difference within Europe are thousands of years old, as mentioned in Roman texts. IQ differences are likely significantly more recent. Of course, thousand of years ago Southern Germany already had harsher climate, but population movement since then obscured the temperature-height correlation.

However, I think even the IQ gradient is unlikely to be causally connected to climate differences. The cold winter theory probably holds when looking at the entire world or at least continents and the corresponding long time scales. But I doubt that IQ differences that established themselves in the last 1000 years within Europe owe much to climate. Instead I have a different theory: I propose that iodine deficiency had a strong positive selection effect on IQ.

In the Alpine region iodine is scarce. Iodine deficiency can retard IQ by up to 15 points. This effect on the alpine population was so noticeable in the past that in France the term „cretin des Alpes“ was coined (idiot from the Alpes). This entails, that if in an agricultural community a person with an IQ below 70 has little chance of ever supporting a family, this fraction of the population in an iodine deficient region is 7 times larger than elsewhere. So any natural selection for IQ is multiplied by the environmental retardation. This theory also neatly explains the Swiss nobel prize density, which is the highest in the world.