Impact of infancy duration on adult size in 22 subsistence-based societies.
Humans evolved to withstand harsh environments by adaptively decreasing their body size. Thus, adaptation to a hostile environment defers the infancy-childhood transition age (ICT), culminating in short stature. In natural-fertility human societies, this transition is associated with weaning from breastfeeding and the mother's new pregnancy. We therefore used the interbirth interval (IBI) as a surrogate for the ICT. We hypothesized that long IBI will be associated with smaller body size. The sample used is 22 subsistence-based societies of foragers, horticulturalists and pastorals from Africa, South America, Australia and Southeast Asia. The IBI correlated negatively with the average adult bodyweight but not height. After correction for 'pubertal spurt takeoff' and 'weight at age 5', the IBI explains 81% of 'average adult weight' variability. This inter-population study confirms that body weight is adaptively smaller in hostile environments and suggests that the selected trait for this adaptation is the ICT age.