Since women are said to be tougher than men, evolutionary theories suggest those living in poorer communities are predisposed to giving birth to girls to ensure survival of the family line, as men are more at risk of dying younger.
The research team led by social psychologist Dr. Thomas Pollet at Groningen University in the Netherlands says that this reveals a "previously undetected form of sex-ratio biasing in humans."
During the study, the researchers looked at 95,000 Rwandan mothers in either monogamous unions, involving a single couple or polygynous marriages, a form of polygamy, where the man has several wives.
The low-ranking wives in polygynous marriages are believed to be worse because they get a smaller share of their husband's resources.
The researchers found that third or lower-ranking wives had 106 daughters for every 100 sons, compared with those in monogamous marriages who had 99 daughters for every 100 sons.
"Our findings show that low-ranking wives, of third order or lower, have lower fertility than other women, suggesting that they are in poorer condition," the Telegraph quoted Pollet as saying.
"These low-ranking wives have relatively more daughters than higher-ranking and monogamously married wives.
"Mothers in poor condition, here lower-ranking co-wives in a polygynous marriage, may overproduce daughters because these give them greater fitness returns than sons," he added. For example, black mothers in America have long born fewer boys than white mothers who have seven more for every thousand births.
The study also has implication for countries like India and China where female infanticide occurs, as the findings suggest these countries cannot avoid having an excess of daughters.
Thirty years ago, Robert Trivers, an evolutionary biologist, and Dan Willard, a mathematician had argued that strong healthy women tend to have sons in order to ensure her genes and family line are passed on.
These sons are in turn strong and outdo other weaker male offspring to reproduce, thus ensuring survival of the fittest and of the family line.
But weaker or poor mothers tend to have weaker sons who do not do well and are more likely to die early, earlier than women in the same society.
To ensure survival of the family line, the women tend to have daughters because they are more likely to be survive long enough to become parents themselves.
The study appears in Biology Letters.