Evolutionary parental investment theory predicts that parents invest preferentially in offspring best able to translate investments into fitness payoffs. It has also been proposed that where the reproductive prospects of offspring are directly correlated with parental investment and variance in fertility is higher for males than females, parents in better condition should bias investment toward males while those in poorer condition should bias investment toward females. Lactation is arguably among the costliest forms of investment expended by mothers and is thus expected to be allocated in ways consistent with fitness payoffs. Quantitative data collected among 110 Papua New Guinean mother-infant pairs during 470 h of focal follows on nursing frequency and duration and responses to infant demands by maternal and offspring characteristics are presented to provide empirically-based descriptions of infant care and tests of evolutionary parental investment theory. Results indicate that mothers show very high levels of investment in offspring. However, although breastfeeding in developing countries is often characterized as on-demand, fussing and crying by infants were only attended to with breastfeeding about 30% of the time. Contrary to expectations of parental investment theory that parents should invest less in poorer quality offspring, mothers increased investment in offspring in poorer condition. The expectation that mothers in better condition would bias investment toward male offspring was also not supported; better nourished mothers biased investment toward female offspring. This study illustrates how infant feeding data may be used for testing larger evolutionary questions such as those derived from parental investment theory. Am. J. Hum. Biol. 2009. © 2009 Wiley-Liss, Inc.