The study, conducted by researchers from the Faculty of Biology and Environmental Protection of the University of Lodz, has been published in the American Journal of Human Biology.

“If the fourth (ring) finger is longer than the second (index) finger, we are talking about greater exposure to testosterone. The reverse situation indicates greater exposure to oestrogen,” explained Paulina Pruszkowska-Przybylska, PhD, the study’s first author.

Researchers examined 182 adults — 106 women and 76 men — aged 30 to 76, with an average age of 51.5 years. Finger lengths were measured, and blood samples were used to analyse DNA methylation.

Scientists calculated participants’ epigenetic age using so-called epigenetic clocks — algorithms that estimate biological ageing based on DNA methylation patterns.

These clocks are also used to predict disease risk and lifespan.

2D:4D, OR HORMONES ON HAND

A statistically significant association was found between a higher 2D:4D ratio on the left hand and accelerated epigenetic ageing in men. No such effect was observed in women.

“Of course, male foetuses develop in an environment with higher testosterone levels than female foetuses, but the exact hormone ratios may vary regardless of gender. They are influenced by the environment in which the mother lived during pregnancy and her individual health,” said Pruszkowska-Przybylska.

“For example, the mother's diet, especially the consumption of meat containing hormones, can modify the sex hormone ratios in amniotic fluid. Some scientists believe that phytoestrogens may have a similar effect, although research results are inconclusive. The pregnant woman's possible exposure to chemicals in the workplace is also important,” she added.

The final balance of sex hormones in amniotic fluid — shaped by maternal health, diet, environment, and the foetus’s sex — plays a role in later physical and biological traits.

“If testosterone dominates, a person (regardless of sex) has a greater tendency to develop muscle tissue in the future, which is associated with greater muscle strength and better athletic performance,” Pruszkowska-Przybylska said.

“Of course, 2D:4D — like other biological indicators — does not provide 100% certainty regarding individual characteristics and predispositions, [but] it does allow to identify certain statistical dependencies between prenatal developmental conditions and physical, health, and behavioural characteristics later in life.”

WHO WILL AGE FASTER?

The second part of the project focused on estimating biological age using DNA methylation, a chemical process that affects gene expression without changing the underlying DNA sequence.

“It can translate into phenotypic characteristics such as wrinkles, greying, or decreased skin firmness, although it is certainly not the only factor influencing these processes, as genetic predispositions also play a significant role,” said Pruszkowska-Przybylska.

Among the various epigenetic clocks used in the study, two models — PhenoAge and DNAmTL — showed statistically significant results.

PhenoAge helps predict the risk of diseases like cancer and coronary heart disease.

DNAmTL is a biomarker of telomere length, which shortens with age and is linked to cardiovascular disease, cancer, and immune system ageing.

In both models, a higher 2D:4D ratio in men correlated with faster epigenetic ageing. No such pattern was detected in women.

“We noticed a certain correlation. To conclusively confirm it, research would need to be conducted from the moment of pregnancy, with hormone measurements in amniotic fluid, finger length assessment after birth, and measurement of epigenetic age in subsequent years, which is associated with many methodological and ethical difficulties,” she said.

“The idea is to identify and minimise factors that may increase the risk of accelerated biological ageing already during foetal life,” Pruszkowska-Przybylska concluded.

PAP - Science in Poland, Katarzyna Czechowicz

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