Evidence for continuing secular increase in height within families in the United States

Measures of mature heights of the second generation of longitudinal subjects in the Fels Institute growth study are now becoming available. When augmented by the heights of the parents who enrolled the first-generation children, these families provide three-generation data for mature height. Heights of the first-generation spouses are also available. The successive generations correspond approximately to the following birth cohorts: 1886-1920, 1921-1949, and 1950-1968. Also available in the Fels data are heights for two-generation families with parents and offspring in successive cohorts. In the three-generation families, the mean increases (cm) for 34 grandmothers, mothers, and daughters were 2.99 and 2.77, both significant with P < 0.01; for 14 grandfathers, fathers and sons, 3.15 and 3.63, the latter significant with P < 0.05. In the first and second cohort two-generation families, the mean increase for 67 mothers and daughters was 4.01 (P < 0.01); for 62 fathers and sons, 3.84 (P < 0.01). In the second and third cohorts, the corresponding figures for 53 and 52 pairs were 3.00 and 4.36, both P < 0.01. In every case, the change in the rate of increase between the first and second generation cohort, and that between the second and third, was not statistically significant. There is no clear evidence that the secular increase in height is decreasing in these families. The finding must be qualified, however, by the observation that most of the first-generation subjects grew to maturity during the Great Depression, when nutrition may have suffered relative to that of the post-World War II years of the maturing second generation. Study of a third generation of Fels subjects may be necessary before a decrease in the secular trend becomes evident in this sample.