However, this assertion flies in the face of evolutionary logic. It is only by investing in their children that parents reap the evolutionary benefit of reproduction; evolutionary models pretty much demand that parents put more into having children than they get back. Hillard Kaplan:
In contrast to wealth-flows theory, models of fertility and parental investment derived from evolutionary biology expect that the net flow of resources will always be from parents to offspring, even when fertility is high. The logic underlying this expectation is that natural selection will have produced a preponderance of organisms that are designed to extract resources from the environment and convert those resources into descendants carrying replicas of their genetic material. Organisms that extracted a net gain from offspring would produce fewer genetic descendants than those that utilized their own labor and excess energy to produce more viable offspring. This does not mean that natural selection could not favor a positive flow from some offspring to parents or from offspring to parents at some ages but that the overall intergenerational flow of resources will be downward.Kaplan backed up this assertion with very detailed data from three hunter-gatherer societies, finding that in every case parents got less back from the time they invested in raising and teaching children than they would have if they had spent that time hunting, gathering, or doing other work. In simplest terms, children eat much more than they catch, and this continues right up to the time that they have their own children. Eva Mueller found the same for peasant societies:
Children have negative economic value in peasant agriculture. Up to the time that they become parents themselves, children consume more than they produce.Nor is there much evidence that raising children provides care in old age; in the first place, few people in traditional societies lived much past the end of their working lives, and in the second studies have shown that aging parents could hire caregivers for much less than the cost of raising children. Peasants, for example, could invest their profits in buying land, or lend out money at interest, rather than have children, and they would come out economically ahead.
In the past, people had children for the same reason we do: the emotional imperatives built into us by evolution.
This is from a fascinating review article on the economics of family life by Ted Bergstrom, which you can read here.