Losing Weight
The genetic factors which must be added to this but which the authors believe are of secondary importance are, as you suggest, heterosis, and possibly the immeasurable one of whether the genotype of the child who now survives because of improved maternal care is the same from the point of view of physique as that of the children who would have been sturdy enough to survive anyway.
Abstract
to accelerate for environmental reasons alone; indeed, this may be an underestimate, for mortality data are a very crude measure of maternal efficiency and acceleration may continue after infant mortality has reached an irreducible minimum. The genetic factors which must be added to this but which we believe are of secondary importance are, as you suggest, heterosis, and possibly the immeasurable one of whether the genotype of the child who now survives because of improved maternal care is the same from the point of view of physique as that of the child who would have been sturdy enough to survive anyway.