Scientists from the Guizhou University of Traditional Chinese Medicine (China) studied how the age of first sexual intercourse is linked to depression in adults. The work was published in The Journal of Sexual Medicine. The researchers used two independent approaches - analysis of surveys and genetic data.
At the first stage, they took data from the US National Health and Nutrition Examination Survey (NHANES) for 2013-2016. The sample included 4,907 adults, roughly half of them women. Participants reported their age at first sex themselves and completed a standard questionnaire to assess symptoms of depression - a condition manifested by persistent low mood, loss of interest and other impairments.
It turned out that the later a person began sexual activity, the fewer depression symptoms they had. This inverse relationship (that is, when one indicator increases, the other decreases) was seen in both sexes, but was noticeably stronger in women. When the researchers adjusted the calculations for income, education and health characteristics, the link disappeared in men but remained in women.
In addition, the relationship turned out to be nonlinear: depression symptoms declined smoothly with increasing age at first sex up to about 17 years, then leveled off and rose slightly among those who started even later. In women this curve persisted even after adjustments for socioeconomic factors, while in men it did not.
In the second part, they used Mendelian randomization. This is a method that uses genetic variants randomly distributed at birth to distinguish causal influence from simple correlation - a statistical association that by itself does not prove causation. The scientists combined data from genome-wide association studies (GWAS), which compare the genomes of hundreds of thousands of people to find small genetic differences associated with traits.
Genetic predisposition to a later start of sexual activity was associated with a lower risk of depression - both overall and severe. But when genetic predictors of income and education were added to the model, the direct link almost disappeared in both sexes. In other words, a significant part of the effect ran through socioeconomic factors: later sexual debut was more often combined with longer schooling and higher income, and this, in turn, is associated with less depression. In women, the duration of education also played a role, and in both sexes, income played a role in severe depression.
The authors note limitations. The genetic markers used to predict age at first sex may also reflect more general behavioral or social characteristics, such as a tendency toward caution or long-term planning. Survey data depend on the accuracy of participants' memories, and the genetic part is based mainly on people of European ancestry, so the results may not transfer to other populations. The stronger links in women point to social factors that were not measured: coercion, inequality in relationships and gender-based violence. These mechanisms require further study.