IQ increased throughout the 20th century, but this trend has recently slowed down or even reversed in a number of countries. The causes of the rise and fall of IQ appear to be largely environmental, but we have little idea what those causes might be.
The Flynn effect, a rise in intelligence over the 20th century, and its recent reversal, are based on environmental causes, but their nature remains a puzzle.
By the way, here's the (really cool) paper showing an IQ rise followed by an IQ decline in Norway. It argues both are almost completely caused by (as-yet-unknown) changes in the environment:
Flynn effect and its reversal are both environmentally caused
Flynn effect reversal environmental says
IQ is apparently declining steadily in a very broad-based way. In PNAS "Flynn effect and its reversal are both environmentally caused"
Really cool study (though that it's environmental was already so well accepted, given speed it happened, that it underlies a lot of arguments about whether various gaps are): "Flynn effect and its reversal are both environmentally caused"
Flynn effects (increases in IQ) and their reversal in Norway occur within families (In almost all Norwegian males from conscription), suggesting parental behavior and/or cultural forces (different sibling behavior) as the main driver. from
Another intriguing fact is that some countries are seeing a negative Flynn effect in cohorts exposed to more education: It appears that not all education is the same.