There's plenty of evidence from any number of animal species that, when times are hard, parents prefer daughters over sons. There's also human evidence that, in similar circumstances, breast milk is involuntarily channelled so that daughters receive richer milk than sons. This channelling of nutritional resources obeys an evolutionary rule that daughters represent a better investment in hard times because they are more certain to produce children than poor sons who may be un-attractive as mates and remain childless.
This study, by researchers at the Carlson School of Management and Rutgers Business School, extends this line of thinking to the material resources parents dole out to sons and daughters dependent upon the economic climate. They found that "participants preferred to enroll a daughter rather than a son in beneficial programs, preferred to give a U.S. Treasury bond to a daughter rather than a son, and bequeathed a greater share of their assets to female offspring in their will when they perceived economic conditions to be poor.
As this article says: "Almost all parents say that they don't favor one of their children over another, but economic recessions subconsciously lead parents to prefer girls over boys," said Rutgers professor of marketing Kristina Durante, lead author of the study." While Professor Vladas Griskevicius of the Carlson School added: ""These findings in humans align well with the behavior of other animals. When resources are scarce parents prefer females because they have a larger reproductive payoff. Almost every female child will produce some offspring, but many male children end up having zero offspring."