Dahlberg's ('43) hypothesis that heterosis is a major component of the secular trend towards increased stature is supported by the significant negative correlation between stature and the coefficient of inbreeding in 70 departments of France. This phenomenon is especially evident after the onset of puberty, (r at age 7 = −0.25 for boys: −0.24 for girls; but r at age 14 = −0.44 for boys and −0.56 for girls) and is found also in adults.
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Eugène Schreider (1967) studied this question.