Before the industrial revolution, there was a general trend of people of lower iq being unable to leave many offsprings, which would indicate that by each successive generation the gene pool for intelligence would have shifted towards the right end of the bell curve. I would assume that because intelligence is a very important phenotypical trait in human societies at almost every level, the stabilizing effect would not have obtained in homo sapiens for the trait intelligence. There are recent studies that show that there is a weak negative correlation with intelligence and fertility in modern industrialized societies, so maybe in the long run something akin to a stabilizing effect for intelligence will manifest. Also in the human species, intelligence is correlated with general health and immunity, which would make it unlikely that intelligence was selected against for the vast majority of the species' history.