I am the author of a study published in PLOS ONE in 2013 which follows up on this article by Cameron/Dalerum:
http://www.plosone.org/ar...

In this follow-up study, I also use data from the Forbes list of billionaires, in this case the Forbes 400 list of the year 2009. Through a meticulous search of information on billionaire offspring in newspaper archives and on the Internet in which I had help from a team of research assistants, I was able to reduce the percentage of missing information considerably in comparison to the study of Cameron/Dalerum.

This has effects on the estimated effect: I find a much smaller effect of wealth on offspring sex composition at birth which is also conditional on the timing of wealth acquisition. In the article, I argue that the smaller effect may be due to reduced sample selection (male offspring is more likely to appear in newspaper and Web sources). The remaining effect of an increased probability for male births is conditional on the timing of wealth acquisition: it only occurs among male heirs (who had acquired their wealth prior to conception of all of their children), but not among male self-made billionaires (who had not yet acquired their wealth prior to conception of all of their children).