In the days of old many kids didn’t make it to adulthood: say 40% among hunter-gatherers. To a a degree, this was caused by genetic load. High mortality purged some of that genetic load, especially to the extent that selection took the form of truncation selection. This process kept mutational load in equilibrium.
In the past century or two, this mortality has become much lower – so this form of selection has become weaker. Mutational load must be increasing. How fast? We will probably know quite soon, from sequencing recent and contemporary individuals. We’ll probably have a handle on the phenotypic impact as well.
In the meantime, we can get a rough idea of the impact of relaxed selection by looking at pioneers, populations that have expanded rapidly in the past few centuries. Some of those populations had a much higher fraction of kids survive than the typical long-term human average: in New England, after he first few years, something like 75% of kids made it to adulthood. This went on for a long time, something like a couple of hundred years – which is why 20,000 Puritan settlers have around twenty million descendants today. Similar things happened with the Quebecois. We know that this relaxed selection among pioneer types left a higher burden of Mendelian disease – not incredibly high, but higher than in similar groups that didn’t undergo a rapid expansion.
How much did this drop IQ? Can’t have been much.