> We did, however, find the average maternal age per country (N = 7) to negatively correlate to the level of IQ decline (r = −0.88). Despite the small sample si

> We did, however, find the average maternal age per country (N = 7) to negatively correlate to the level of IQ decline (r = −0.88). Despite the small sample size, the correlation reached significance (p < 0.01). Nevertheless, as the correlation was negative this seems to indicate that among the countries showing a negative Flynn Effect, the ones with the highest average maternal age show smaller declines in IQ. Anyone else see the problem here? They are comparing the change in a variable over time with the value of another variable at a fixed point in time. That means they haven't canceled out the time variable, resulting in a confound with the change in maternal age. Would it be possible to get their result--a negative correlation between greater maternal age at the end of a time period and a drop in IQ over the whole time period--if, in fact, the correlation between maternal age and lower IQ is positive? Yes. One way this could happen is if the relationship between maternal age and lower IQ is linear but the rate of change in maternal age over time is nonlinear and decreasing. The latter is quite plausible because fertility already decreases with age in a nonlinear fashion. It's easier for a country's average maternal age to go from 25 to 35 than from 35 to 45 without even taking societal factors into account. The authors admit that their finding is contrary to other research they reviewed which found maternal age to be positively associated with lower IQs. My guess is that the discrepancy is because of the authors' flawed model. Edit: typo, more quote