Having siblings costs you money over the course of a lifetime, just ask Ohio State sociologist Lisa Keister.
How much? Holding a variety of relevant factors constant, individuals with one sibling have an average net worth of $62,000. If you have another sibling, it drops to $49,000, then to $40,000, then to $24,000. Seven or more siblings, you have on average net worth of $6,000. Keister suggests, consistent with the research of Gary Becker, that parents invest less in each child when they have more children.
Keister herself has three brothers. See this summary of the research.
I am surprised that the effect from one child to two, or from two to three, is so small. And how much stems from a change in parental investments? Some fraction of the parents with more kids did not expect the pregnancies but rather made planning mistakes. If you think that planning/execution capabilities are carried in the genes, you would expect the offspring to be worth less for genetic reasons. So the Becker effect may be weak rather than strong, at least for the first few children.
Keister’s research also argues that much of the black-white wealth gap is due to the lesser willingness of African-Americans to put their money into stocks and mutual funds. It has long been a puzzle to economists (see here for one account) why individuals do not invest more money in the stock market, given that American stocks have outperformed bonds by significant margins over all previous twenty to thirty time horizons. Keister’s research both deepens the puzzle and hints at the relevance of psychology and context for understanding investment decisions.