New study finds a causal link between sexual activity early in life and major depressive disorder

In a recent study published in Journal of Affective Disorders, researchers sought to understand whether there is a cause-and-effect relationship between certain sexual behaviors and major depressive disorder (MDD). They focused on two sexual factors: early intercourse and the number of sexual partners a person has had.

The results revealed a causal effect of early intercourse on MDD, suggesting that delaying the age at first intercourse may have a protective effect against the development of depression. These findings contribute to our understanding of the potential link between early sexual behavior and major depressive disorder.

Previous research has suggested a link between early intercourse and MDD and other mental health problems. When people have sexual encounters between the ages of 12 and 14 with two or more partners, the risk of later mental illness increases. But the reason for this relationship is still unclear. It may be that those predisposed to mental illness are more likely to engage in early sexual activity, the early sexual activity may make individuals vulnerable to mental illness, or there may be other as yet unknown factors that influence both.

Conducting large-scale cohort studies can be expensive and time-consuming, and it remains unclear whether risky sexual factors cause or result from MDD. Therefore, alternative methods are needed to strengthen the understanding of causality before expensive trials are considered.

For their new study, Zhe Lu and colleagues used publicly available databases, specifically the UK Biobank and the Psychiatric Genomics Consortium. To ensure population homogeneity, only data from the European population were considered. Genetic predictors of sexual factors and MDD were obtained from these databases.

In addition, genetic predictors for five risky behaviors (self-harm, suicide attempt, psychoactive drug use, alcohol use and tobacco use) were also obtained from the UK Biobank. The sample size for the analysis of age at first intercourse was 406,457 individuals, and the sample size for the analysis of lifetime number of sexual partners was 378,882.

The researchers used a technique known as Mendelian randomization, a method used in epidemiological research to examine causal relationships between an exposure and an outcome by exploiting genetic variants as instrumental variables. Mendelian randomization uses genetic variants from genome-wide association studies as instrumental variables to demonstrate causal relationships between exposure and outcome.

Data analysis revealed a significant causal effect of sexual behavior on MDD. Early intercourse and having multiple sexual partners increased the risk of MDD. These effects were robust across different analysis methods. The analysis indicated that each additional year of delay in age at first intercourse was associated with a 6% reduction in the risk of MDD.

The research had several strengths, including the use of large genetic data and careful consideration of potential confounders. However, several limitations should be considered. First, the study focused on the European population, which limited the generalizability of the results to other populations. Second, the genetic analysis assumes that genetic variants are valid instrumental variables, which may not always hold.

Third, the study relied on self-reported data for sexual factors and MDD, which introduces the possibility of recall bias and measurement error. Finally, the study did not account for potential confounding factors influencing the relationship between sexual factors and MDD, such as socioeconomic status, childhood trauma, or other mental disorders.

Despite these limitations, this study contributes to the growing body of research on the relationship between sexual factors and mental health outcomes. Identification of a causal effect of early intercourse on MDD suggests that interventions aimed at delaying the age at first intercourse may have potential benefits in preventing or reducing the risk of MDD.

The study emphasizes the importance of addressing depression among sexually active teenagers and the potential role of risky behaviors as mediators in this relationship. The results have implications for the prevention and management of MDD.

The study, “Identifying causal relationships between early intercourse or number of sexual partners and major depressive disorder: A two-way two-sample Mendelian randomization analysis,” was authored by Zhe Lu, Yaoyao Sun, Yundan Liao, Zhewei Kang, Xiaoyang Feng, Guorui Zhao, Junyuan Sun, Yuyanan Zhang, and Weihua Yue.