Non-economists cringe when they hear us talking about investment in children and the quantity/quality trade-off in this regard. Yet, this is a very real aspect of child rearing pointed out by Gary Becker that is at the core of many models, and has been found wild in nature. This trade-off is though to be an integral part of the demographic transition, where fertility suddenly drops massively in the course of development.
Marc Klemp and Jacob Weisdorf look at data from Anglican parish registers from the 18th century that contain all sort of demographic data to look at the child quality/quantity trade-off during the Industrial Revolution. Theory tells us that if the returns to education and/or the cost of time (wages) get larger, parents switch from having many children with no education to few of them with better education. Klemp and Weisdorf's data indicates clearly that this trade-off is present: each additional sibling reduces by 8% the probability of a child eventually becoming literate. That is a strong effect, in particular considering the larger number of children at the time, and its rather large standard deviation during this time of transition.