When times are bleak and the future looks grim, parents instinctively favour their daughters, because they have the best chance of helping the family — and the human race — to survive by having babies, researchers argue in a new study.

"Almost all parents say that they don't favour one of their children over another, but economic recessions subconsciously lead parents to prefer girls over boys," said Rutgers Business School professor of marketing Kristina Durante, lead author of the study in the Journal of Consumer Research.

"These findings in humans align well with the behaviour of other animals," Professor Vladas Griskevicius of the Carlson School of Management at the University of Minnesota tells Science Daily. "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."

In the study, test subjects were given different news accounts projecting different economic situations from bright to grim. They were then asked to divvy up their assets between an imagined son and a daughter. When times were good the split was 50/50. When times were bad the division favoured the daughter 60/40. The difference was even more more pronounced as the son and daughter neared reproductive age.

The researchers also examined the relationship between US Real Gross Domestic Product and retail spending on apparel for boys and girls between 1984 and 2011. They discovered that when the economy was struggling, the ratio of spending on girls versus boys increased 19.8% compared to when the economy was faring well.

The love distribution by parents in tough times — what the study calls a "reproductive risk-management strategy" — appears calculating in the study but is largely unconscious, researchers believe.

"Investing in male offspring is akin to betting on a long shot in a horse race: males can yield a huge payoff but often they will yield nothing. Investing in female offspring is akin to betting on the heavy favourite: the winning payout is not large, but there is a good chance of some winnings that can subsequently continue to build through bets on future races," the study explains.

"As such, resource-scarce conditions should encourage greater investment in girls because investing in female offspring has historically represented a safer and more certain option for continuing one's genetic lineage."