Consider height, for example. It's clear that there has been a genuine, real, and sustained increase in height over the last century among all groups in industrialized countries. Yet this increase has not eliminated the ethnic gaps; the improvements in those aspects of the environment affecting height (nutrition, healthcare, etc.) have acted as a rising tide lifting all boats.Height is actually a very good analogue for IQ, since height, like IQ is a measure of good development of an organism, and its secular increase tracks IQ increases fairly closely.
In Italy, all regions have increased in height in the last century and a half, but trends were not uniform:
The height of young men has increased in all regions of Italy. The secular trend and the regional changes in stature are correlated with economic growth and a general improvement of living conditions. This is suggested by the relationship between height and various socio-economic indicators. A comparison of the 1927 birth cohort with the 1980 birth cohort shows that the mean heights for populations in Italy's southern areas, which were shorter than the national average in 1927, underwent the largest increases.In Europe as a whole, secular increases in height vary greatly, in the direction suggested by the Italian data.
The narrowing of the gap is also evident in England:
This is the first time that trends in growth by ethnic group have been quantified in England. Despite the relative deprivation of the ethnic minority groups in inner city areas, their mean trends in height were greater than those of the representative white sample, and comparable to those of the inner city white children.Similar results occur in the United States:
Overall, black children experienced much larger secular increases in BMI, weight, and height than did white children.So, all boats may be rising, but they are not rising at the same rate. Hence, the difference in height decreases, at the same time that absolute height values increase.
PS: We should be careful when we bring height into discussions of human intelligence. In height, unlike intelligence, it is reasonable that there may be substantial genetic differences between populations. Differences between animal size and shape in different climates are well-established in zoology, and it is unlikely that man would be an exception in this case. Unfortunately, we cannot use our zoological insight to address the intelligence issue, since man is the only cognitively advanced species on the planet.