A new scientific review challenges the widespread idea that certain physical features in women serve as biological signals of high fertility. The comprehensive analysis suggests that characteristics often described as feminine, such as a specific waist-to-hip ratio or voice pitch, do not reliably predict how many children a woman will have. These findings appear in the journal Evolutionary Human Sciences.
Evolutionary biology often distinguishes between male and female physical traits within a species. Beyond the primary reproductive organs, humans exhibit differences in body fat distribution, voice pitch, and facial structure. Women, on average, possess a lower waist-to-hip ratio and a higher voice pitch than men. They also tend to have distinct facial features, such as fuller lips and smaller chins.
A common explanation for these differences involves sexual selection. This perspective proposes that men evolved preferences for specific female traits because those traits indicate high reproductive potential. If this hypothesis is correct, women with more pronounced feminine features should biologically be capable of producing more offspring. This concept suggests that beauty standards may have roots in biological utility.
“Evolutionary researchers have long suggested that heterosexual men are attracted to ‘feminine’ traits in women – like feminine facial features, a curvy body, and a narrow waist – because these traits might signal higher fertility,” said study author Linda Lidborg, a postdoctoral research associate at Durham University.
“Similar patterns have been proposed in men for masculine traits, though in men testosterone and other factors are thought to play a role for different evolutionary reasons. A few years ago, we studied the link between masculine traits and fertility in men; this new review systematically examines whether women with more feminine traits actually show higher fertility than less feminine women.”
The researchers searched major scientific databases for studies that measured specific physical traits in women and compared them to direct fertility measures. Their review focused on actual reproductive outcomes. These measures included the number of children or grandchildren, pregnancy history, and offspring survival rates. The researchers only included studies that analyzed men and women separately to ensure accurate data regarding female physiology.
The review included data from 19 articles. This encompassed 31 different samples from 16 countries, totaling over 125,000 participants. The physical traits examined included breast size, waist-to-hip ratio, voice pitch, physical strength, and the ratio of the second to fourth finger digits.
The researchers sought data regarding facial femininity but could not find a single study linking facial structure to direct fertility outcomes. This absence of evidence is notable given how often facial femininity is discussed in evolutionary psychology. It remains unknown whether women with more feminine facial features actually have more children.
For waist-to-hip ratio, the review included eight studies. A lower ratio is typically considered more feminine and is often viewed as attractive in many Western contexts. This shape creates the classic “hourglass” figure.
However, the review found that women with a higher, less feminine ratio often had more children. This contradicts the prediction that a lower ratio signals higher fertility. The authors suggest this likely occurs because pregnancy and childbirth physically alter a woman’s body shape, often increasing the waist measurement.
Only one study measured women before they had children and followed them over time. That study found no connection between a woman’s initial waist-to-hip ratio and her later fertility. The data implies that a wider waist may be a result of past fertility rather than a signal of future reproductive potential.
The evidence regarding breast size was similarly inconclusive. Large breasts are often theorized to be a result of sexual selection. Yet, the two articles covering three samples produced conflicting data.
One study found a positive link between breast size and number of children. Another found a negative link, suggesting smaller breasts correlated with more offspring. A third analysis found no connection at all. As with waist shape, pregnancy and breastfeeding affect breast volume, making cross-sectional data difficult to interpret.
Studies on voice pitch provided mixed results as well. One study involving Himba women in Namibia found that higher-pitched voices correlated with higher fertility. Conversely, a study involving Hadza women in Tanzania found no such association. These conflicting findings make it impossible to say whether voice pitch serves as a reliable cue for reproductive potential.
The reviewers also looked at the second-to-fourth finger ratio. This metric compares the length of the index finger to the ring finger. A higher ratio is typically more common in women and is often used as a marker for lower prenatal testosterone exposure.
Across seven studies, the results for finger ratios were inconsistent. While some data suggested a link between a more feminine finger ratio and higher fertility, the effect sizes were very small. Many analyses within these studies showed no significant relationship. The authors noted that this trait is rarely cited as a sexually selected trait in attraction research.
The review also examined physical strength and muscle mass. These are typically traits associated with male competition rather than female fertility. However, the researchers included them to rule out potential mediating effects.
Some data from the Himba sample suggested stronger women had more children. This mirrors findings in men, where strength often correlates with reproductive success. However, other samples showed no such relationship for women.
Overall, the review concludes that the current evidence base is too weak to support the claim that feminine physical traits act as reliable cues for reproductive potential. The popular evolutionary narrative that men prefer these traits because they signal fertility lacks robust empirical support.
“Current evidence does not show that women with more feminine traits are more fertile than those with less feminine traits,” Lidborg told PsyPost. “This doesn’t mean the link doesn’t exist – it means that, so far, there isn’t evidence to support it. In our article, we advise caution in repeating this hypothesis, since there isn’t sufficient evidence for it at this point.”
Several limitations affect the certainty of these conclusions. Most of the reviewed studies used a cross-sectional design rather than a longitudinal one. This means they measured physical traits and the number of children at the same time.
This design makes it difficult to determine the direction of causality. It is unclear if a physical trait leads to higher fertility or if bearing children causes changes in the physical trait. This is particularly relevant for body shape and breast size, which change significantly after pregnancy.
Additionally, many studies relied on participants from industrialized nations where contraception is widely used. The widespread use of birth control obscures natural associations between biology and reproductive outcomes. In these societies, family size is often a choice rather than a reflection of biological capacity.
Some samples were also too small to detect weak statistical relationships. If a trait provides only a very small advantage in fertility, a study needs a large number of participants to prove it. The authors caution that some non-significant results might simply lack the statistical power to show an effect.
The authors suggest that future research should prioritize longitudinal designs to address these gaps. Ideally, researchers would measure the physical traits of women before they begin having children and track their reproductive history over their lifetimes. This would separate the signal of fertility from the physical aftereffects of pregnancy.
Examining populations that do not use modern contraception would also provide clearer data on biological fertility. Comparing results across different cultures and ecological contexts is necessary to understand if these traits are universal signals.
“There are many reasons why men and women have evolved different faces and bodies, and attraction is only one possibility,” Lidborg said. “Some traits, like wider hips, are needed for childbirth, while others – like a more feminine facial shape – don’t have a clear reproductive function, so their evolutionary origins are less certain. Our group continues to study the evolutionary pressures that may have shaped human physical appearance.”
The study, “A systematic review of the association between women’s morphological traits and fertility,” was authored by Linda H. Lidborg and Lynda G. Boothroyd.
