From a biological point of view, this makes sense, as the Earth is at a slight tilt, putting the hottest places on the planet just South of the Equator, which sure enough shown on a global IQ map by country have inhabitants with the lowest average test scores.

However, the United States is a land of immigrants, and the South simply has more people from races that evolved South of the Equator (hispanics and african-americans) which lower the score. The Northern states in the country are primarily caucasian, and they score higher on average, on other tests as well such as SAT scores.