Secular trends in human growth, maturation, and development.

Rates of growth during childhood have increased considerably during the past 50-100 years. Because they are associated with increased rates of maturation, these size increases are maximal at ages when recently measured groups are pubescent but those measured in the past had not reached pubescence. Large secular increases in rates of growth and maturation have occurred in all developed countries but not in many other countries. There have been secular increases in recumbent length at birth in Italy and France but little change in the United States. The secular increase in childhood stature is much more obvious, being about 1.5 cm/decade for 12-year-old children, although for young adults the secular increase in stature has been about 0.4 cm/decade in most developed countries. In the United States these trends have been similar for blacks and whites during childhood but greater for black men than white men. Similarly, secular trends in birth weights have been small, but there have been large trends for children (about 1.5 kg/decade for 12-year-olds). These secular changes in size have been associated with an acceleration of maturation that is most evident in the 0.3 years/decade advancement in age at menarche. This advancement has now ceased in Norway and England. The changes in body proportions during recent decades are less marked than those in body size. Leg length, particularly the length of the thigh, seems to have increased more than stature in men but not women; chest circumference has increased more rapidly than stature in each sex. The relationships between stature and weight have changed in different ways in various national groups. Similarly, the relative changes in head length and head breadth vary with the groups studied. Few sets of data allow conclusions about possible secular trends in body composition, but subcutaneous fat thicknesses have increased, especially at the upper percentiles. Also strength, which reflects muscle mass, has increased absolutely, although it has decreased relative to stature. Undoubtedly the secular trend is due to various factors; the identification of causes is necessarily speculative. Changes in nutrition alone could not account for the trends which exceed the original socioeconomic differentials. In the United States, there have been per capita increases in the intake of protein and fat from animal sources, decreases in carbohydrates and fat from vegetable sources, and little change in caloric intake. It is not clear that these changes constitute better nutrition. The secular trends could reflect environmental improvements, specifically changes in health practices and living conditions leading to improvements in mortality rates and life expectancy. These factors are interrelated with those concerning family size. Also genetic factors, especially heterosis, may have played a small role in causing the secular trends...