Given enough tough, it can happen. Its a factor that will push evolution, as one selective trait continues to progress at a higher rate than the other trait the mean will lean toward one side and continue to grow in that direction. It's simple math really. Regardless of theories suggesting that the average IQ will always rise - and the Lemarkian theories that what took us so long to learn will take our kids no time at all to master - is only true to those who have those desired traits. But as you look at the standpoint of lower IQ genes reproducing faster, they begin to out number the high IQ genes thus becoming selected.

Now, unless someone wants to play hitler and start a new round of eugenics, this trait could possibly continue for a long time. At least in the U.S.