The ‘2D:4D’ ratio: How a child’s finger length could be indicative of mom’s income level

SWANSEA, United Kingdom — A new study finds a parent’s income level and her child’s hormone levels in the womb may go hand-in-hand. Researchers from Swansea University report that babies born to low-income mothers tend to have longer index fingers than ring fingers. Conversely, kids born to high income moms usually have longer ring fingers.

What does any of that have to do with hormones? Well, a longer index finger is a female characteristic indicating higher estrogen levels and a longer ring finger is generally a male trait synonymous with high testosterone levels. Researchers theorize that low-income mothers feminize their children in the womb and high-income mothers masculinize their kids.

As far as why, researchers say the apparent relationship between mom’s income and hormone levels is likely an unconscious evolutionary response on the mother’s part. Its intention may be to give the children the best odds of reproducing one day.

Estrogen versus testosterone

fingers hand diagram
The study was based on the relationship between the length of a person’s index and ring fingers, known as the 2D:4D ratio. (Credit: Swansea University)

Scientists refer to the relationship between the length of a person’s index and ring fingers as the 2D:4D ratio. Its implications have been the subject of scientific debate for some time. Researchers say over a thousand studies have examined the 2D:4D ratio, but this new report sets itself apart by focusing specifically on the ratio in regards to parental income before birth.

“Our results show that mothers with high income may secrete high levels of testosterone relative to estrogen early in pregnancy, thereby masculinizing their male and female children. In contrast, women with low income may secrete low levels of testosterone, which will feminize their male and female children. 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,” says study leader Professor John Manning in a university release.

“For high-income mothers, the advantages of high testosterone for their sons are likely to outweigh its disadvantages for their daughters. For low-income mothers, the fitness gain from feminized daughters is likely to outweigh the fitness loss for feminized sons.”

Is the 2D:4D ratio really reliable?

Researchers theorized before carrying out this study that sons born to high-income families have a better chance of reproducing themselves. On the other hand, daughters born to low-income families represent the best chance of starting a family one day.

To test their hypothesis, study authors analyzed data on over 250,000 people from roughly 200 countries. Each participant measured their personal 2D:4D ratio and reported their parents’ income.

Across the board, among both male and female study participants, those born to high-income parents reported longer ring fingers. Those born to poorer households reported longer index fingers.

“These patterns suggest important effects on public health which are linked to poverty. Low testosterone and high estrogen in male fetuses may predispose those men, as adults, to diseases linked to poverty such as heart attacks, strokes, and high blood pressure. It is well known that poverty is closely associated with poorer health. What our research indicates is that this link can be replicated across generations,” Prof. Manning concludes.

The study is published in the Journal of Biosocial Science.

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