In a surprising discovery that challenges long-held beliefs, researchers have found that some women may have a biological tendency to give birth to children of one particular sex. A large-scale analysis of more than 58,000 women in the United States revealed that genetics, age, and reproductive patterns may subtly influence the sex of offspring. While traditionally seen as a 50/50 chance, the determination of a child’s sex appears to be more complex. The study, led by Jorge Chavarro from the Harvard T.H. Chan School of Public Health, presents a number of intriguing patterns.
- Women who gave birth for the first time after age 28 had 13% higher odds of having only sons or daughters
- Specific genetic variants were found in women who consistently gave birth to children of the same sex
- Larger families showed stronger trends in sex skewing
Table of contents:
- Mothers from the United States show distinct sex ratio patterns
- Genetic variants linked to one-child sex trends
- Cultural decisions complicate biological patterns
- Genetic trends may become clearer in global data
Mothers from the United States show distinct sex ratio patterns
The researchers analyzed data from the long-running Nurses’ Health Study, covering 58,007 women and 146,064 live births between 1956 and 2015. The sex ratios observed deviated from what would be expected by chance, especially among women with multiple children. For instance, in families with three sons, there was a 61% likelihood of having another boy, while families with three daughters had a 58% chance of another girl.
Among two-child families, 53% had one boy and one girl, which is more than expected by a simple 50% probability. These figures suggest that sex distribution among children may be influenced by more than random chance, particularly in larger families.
Genetic variants linked to one-child sex trends
Further genetic investigation involved over 7,000 women from the same dataset. The results showed a link between specific gene variants and the sex of offspring:
- Women with only daughters had certain versions of the NSUN6 gene on chromosome 10
- Women with only sons carried unique variants of the TSHZ1 gene on chromosome 18
These genes are not traditionally associated with reproduction. NSUN6 plays a role in protein synthesis, while TSHZ1 may be involved in the sense of smell. Despite this, the correlation was statistically significant. The findings are consistent with earlier work by Jianzhi Zhang from the University of Michigan, who identified another gene on chromosome 10 associated with a 10% increased chance of giving birth to a girl.
Cultural decisions complicate biological patterns
Not all trends can be explained genetically. Cultural factors play a role as well. Some parents may stop having children after having both a boy and a girl. To adjust for this, researchers excluded the last child from families in some analyses. However, the pattern of sex ratio imbalance persisted even after this adjustment, especially in families with more than two children.
Sex distribution in families by number of children
| Family Composition | Probability of Next Child Being Same Sex |
|---|---|
| 2 boys | 54% chance of 3rd child being a boy |
| 2 girls | 52% chance of 3rd child being a girl |
| 3 boys | 61% chance of 4th child being a boy |
| 3 girls | 58% chance of 4th child being a girl |
Researchers suggest that these patterns might indicate a mix of biological predisposition and conscious family planning. University of Pennsylvania geneticist Iain Mathieson commented that social norms and policies can shape how families are formed. He cautioned, however, that the genetic link to offspring sex remains a hypothesis in need of more evidence.
Genetic trends may become clearer in global data
Experts note that most participants in the study were of European ancestry, which limits the findings’ generalizability. Future studies in more diverse populations will be necessary to validate the results. According to Zhang, the limited size of human families makes it difficult to detect small genetic effects on offspring sex. However, the growth of biobank datasets may help uncover more patterns.
“As more biobanks are being established, more genetic variants influencing human sex ratio at birth may be discovered,” Zhang said.
Despite differing opinions, most scientists agree on one thing: the assumption that every pregnancy has an equal 50/50 chance of being male or female may not always hold true.
Source: Science