Comment by s_r_n

I wonder if the drop in IQ is related to the increase in wealth inequality in these past 50 years as well. (https://en.wikipedia.org/wiki/Distribution_of_wealth)

More cohorts of people are entering the so-called "lower" classes and with this likely comes factors that affect mental aptitude such as decrease in quantity of healthy food consumed and unchallenging jobs.

This would agree with the study, which shows that families' IQs are getting lower with each generation.