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The relationship between weight, height, weight/height2 and fertility is examined in 610 females and 621 males from a 1968 follow-up study of the Third Harvard Growth Study participants. These subjects were born between 1912 and 1918 in the USA. Their physical and mental growth were studied for up to 12 years while they attended public schools in the Boston, Massachusetts, area. Height is not significantly related to fertility in either females or males, but weight and weight/ height2 is positively related to fertility in females (r = +.117 and +.100 respec­tively) and weight/height2 is positively related to fertility in males (r = +.09). Weight and weight/height2 at skeletal age 12 of 305 females are both negatively correlated with later fertility (r = —.102 and - .141 respectively). Thus girls who later had large families were not heavier than average, but in fact, were taller and slimmer. In these data it appears the differential reproduction for heaviness is not likely to have had genetic effects but is probably a secular trend. The fact that taller and slimmer girls later went on to have larger families may be significant for the consideration of sexual selection.