Lower-income mothers secrete more estrogen during the first trimester of pregnancy in order to feminize their children, according to new research in Journal of Biosocial Science. Conversely, higher-income mothers increase their testosterone levels in order to masculinize their kids, all of which results in measurable changes in the length of offspring’s fingers.
The study aimed to test the so-called Trivers-Willard hypothesis, which states that poorer mothers automatically alter their hormone levels during pregnancy in order increase the health of female fetuses at the expense of males, with the reverse being the case for more affluent mothers. According to the authors, this reflects an unconscious evolutionary mechanism that is designed to increase the reproductive success of their children.
Previous research has indicated that testosterone increases the growth of ring fingers, while estrogen boosts the length of index fingers. Hence, men tend to have longer ring fingers than index fingers, with the opposite being true for women.
The relationship between the length of these fingers – known as the 2D:4D ratio – is therefore taken as an indication of prenatal levels of testosterone and estrogen.
To investigate the link between this ratio and family income, the study authors examined the results of a 2005 online survey, in which more that 250,000 people from around the world provided measurements of their ring and index fingers, while also stating their parents’ income.
Results indicated that children from wealthier families tended to have a low 2D:4D ratio, meaning they had long ring fingers. For children of lower-income families, this ratio was generally higher, indicating long index fingers.
These findings suggest that children born to less affluent mothers are typically exposed to higher estrogen levels in the womb, while those born to richer mothers are exposed to more testosterone.
Consequently, sons and daughters of lower-income parents are masculinized, while children of both sexes are feminized if born to higher-income families. Commenting on the significance of this finding, the study authors explain that “high-income mothers will increase the fitness of their sons at the expense of their daughters while low-income mothers will increase the fitness of their daughters at the expense of their sons.”
“Thus, there will be sexually antagonistic effects on the children of both high- and low-income mothers.”
The researchers do concede, however, that estimates of family income were based on participants' childhood memories and could therefore be inaccurate, potentially detracting from the validity of their conclusions. Nevertheless, study author Professor John Manning, from Swansea University, explained in a statement that the results were "consistent with the Trivers-Willard hypothesis."
"This is an evolutionary response, which mothers will not be aware of, let alone able to control. It is geared towards giving their offspring the best chance of reproductive success,” he said.
Taking these findings a step further, the researchers claim that exposure to varying levels of sex hormones in the womb may influence the susceptibility of low-income males to a range of diseases later in life. For instance, it is well established that men from poorer backgrounds have an increased chance of suffering from high blood pressure, diabetes, heart attack and stroke.
The study authors therefore propose that increased estrogen exposure in the womb may be a major driver of these conditions, and that finger length could be a marker of a person’s susceptibility to illness.