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

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

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

Previous research has suggested a link between early sexual intercourse and MDD and other mental health issues. When individuals have sexual encounters between 12 and 14 with two or more partners, the risk for later mental illness increases. But the causal nature of this relationship remains 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 yet unknown factors that affect both.

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

For their new study, Zhe Lu and colleagues utilized 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 for sexual factors and MDD were obtained from these databases.

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Additionally, genetic predictors for five risky behaviors (self-harm, suicide attempts, psychoactive substance abuse, alcohol use, and tobacco use) were also obtained from the UK Biobank. The sample size for the analysis of age at first sexual 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 investigate causal relationships between an exposure and an outcome by leveraging genetic variants as instrumental variables. Mendelian randomization uses genetic variants from genome-wide association studies as instrumental variables to detect causal associations between exposure and outcome.

Data analysis revealed a significant causal effect of sexual behavior on MDD. Early sexual intercourse and having more 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 the age at first sexual intercourse was associated with a 6% reduction in the risk of MDD.

The research had several strengths, including the use of large-scale genetic data and careful consideration of potential confounders. However, several limitations should be considered. First, the study focused on the European population, limiting the generalizability of the findings 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 health conditions.

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

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

The study, “Identifying causal associations between early sexual intercourse or number of sexual partners and major depressive disorders: A bidirectional 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.