Such data are consistent with recent research indicating that slow growth induced by temporary malnourishment can usually be reversed. Chronic underfeeding during childhood, however, permanently affects stature and other traits, including intelligence.
Second, the trend toward increasing height has largely leveled off, suggesting that there is an upper limit to height beyond which our genes are not equipped to take us, regardless of environmental improvements. Interestingly, the age of menarche, which is also influenced by nutrition, has shown a corresponding decrease over this same time period. Some scientists believe that the increase in teenage and out-of-wedlock pregnancies in the developed world may be an unanticipated consequence of improved nutrition.
Third, conditions of poor nutrition are well correlated to smaller stature. For example, the heights of all classes of people, from factory workers to the rich, increased as food quality, production and distribution became more reliable, although class differences still remain. Even more dramatic, the heights of vagrant London boys declined from 1780 to1800 and then rose three inches in just 30 years--an increase that paralleled improving conditions for the poor. Even today, height is used in some countries as an indicator of socioeconomic division, and differences can reveal discrimination within social, ethnic, economic, occupational and geographic groups.
For those hoping that humans might someday shoot basketballs through 15-foot high hoops, the fact that the increase in human height is leveling off no doubt will be disappointing. For those who understand, however, that our genes are merely a blueprint that specifies what is possible given an optimal environment, a limit on height is just one of many limitations in life, and certainly not the most constraining.
With environmental variables perhaps near their optimum, what are the prospects for evolutionary increases in height as a consequence of changes to our genetic blueprints? Apply the methods of the thought experiment above and see.