Children's finger length points to mothers' income level

Low-income mothers feminize their children in the womb by adjusting their hormones, whereas high-income mothers masculinize their children, a major study based on finger length, led by a Swansea University expert, has found.

The phenomenon is an unconscious evolutionary response aimed at boosting their offspring's chances of successful reproduction.

It helps, in part, explain associations between low income, low levels of testosterone before birth, and major causes of mortality such as cardiovascular disease.

The study was based on the relationship between the length of a person's index and ring fingers, known as the 2D:4D ratio. A longer ring finger is a marker of higher levels of testosterone, whereas a longer index finger is a marker of higher levels of oestrogen. Generally, men have longer ring fingers, whereas women have longer index fingers.

The 2D:4D ratio is a widely-debated measure that has been the subject of over 1000 studies, but what is significant about the new report is that the team examined the ratio in relation to parental income.

Led by Professor John Manning of Swansea University, with colleagues in Austria and Jamaica, the team tested a hypothesis about evolutionary influences on the mother and her children. This suggests that for higher-income mothers, sons have higher reproductive success compared to daughters. For lower-income mothers, in contrast, daughters will be more reproductively successful. Known as the Trivers-Willard hypothesis, its senior author, Professor Robert Trivers, was also involved in this new study.

The team used data from over 250,000 people from around 200 countries, who were taking part in an online BBC survey. Participants were asked to measure their index and ring fingers and given instructions on how to do this accurately. They were also asked to indicate their parents' income level.

The results showed:

• Children of parents of above-average income had a low 2D:4D ratio, with longer ring fingers, which indicates high testosterone and low oestrogen before birth, hallmarks of a more masculinized foetus
• Conversely, the children of parents of below-average income had a high 2D:4D ratio with longer index fingers, which indicates lower testosterone and higher oestrogen before birth, markers of a more feminized foetus
• These effects were present for both men and women

Professor John Manning of Swansea University's A-STEM research team in sport science, lead researcher on the study, said:

"Our results show that mothers with high income may secrete high levels of testosterone relative to oestrogen 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.

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.

This pattern is consistent with the Trivers-Willard hypothesis."

Professor Manning explained how the findings could shed light on susceptibility to disease:

"These patterns suggest important effects on public health which are linked to poverty.

Low testosterone and high oestrogen in male foetuses 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".

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The study is in the Journal of Biosocial Sciences, published by Cambridge University Press.

Picture:

Diagram showing the 2D:4D ratio and how it is measured

Credit: John Manning, Swansea University

Notes to editors

Swansea University is a world-class, research-led, dual campus university offering a first-class student experience and has one of the best employability rates of graduates in the UK. The University has the highest possible rating for teaching - the Gold rating in the Teaching Excellence Framework (TEF) in 2018 and was commended for its high proportions of students achieving consistently outstanding outcomes.

Swansea climbed 14 places to 31st in the Guardian University Guide 2019, making us Wales' top ranked university, with one of the best success rates of graduates gaining employment in the UK and the same overall satisfaction level as the Number 1 ranked university.

The 2014 Research Excellence Framework (REF) 2014 results saw Swansea make the 'biggest leap among research-intensive institutions' in the UK (Times Higher Education, December 2014) and achieved its ambition to be a top 30 research University, soaring up the league table to 26th in the UK.

The University is in the top 300 best universities in the world, ranked in the 251-300 group in The Times Higher Education World University rankings 2018. Swansea University now has 23 main partners, awarding joint degrees and post-graduate qualifications.

The University was established in 1920 and was the first campus university in the UK. It currently offers around 350 undergraduate courses and 350 postgraduate courses to circa 20,000 undergraduate and postgraduate students. The University has ambitious expansion plans as it moves towards its centenary in 2020 and aims to continue to extend its global reach and realise its domestic and international potential.

Swansea University is a registered charity. No.1138342. Visit http://www.swansea.ac.uk

For more information:

Kevin Sullivan,
senior press officer,
Swansea University
k.g.sullivan@swansea.ac.uk

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