Identifying causal associations between early sexual intercourse or number of sexual partners and major depressive disorders: A bidirectional two-sample Mendelian randomization analysis

Early sexual intercourse and a greater number of sexual partners have been proved associated with depression. However, the causality of these associations is not clear.To unveil the causal associations between sexual factors and major depression disorder (MDD). The bidirectional, two-sample Mendelian randomization (MR) study was conducted, which used genetic variants associated with two sexual factors (age first had sexual intercourse, n = 406,457; lifetime number of sexual partners, n = 378,882) and MDD (n = 500,199) from the largest genome-wide association studies (GWASs) conducted by the UK biobank and the Psychiatric Genomics Consortium. The two-step MR analysis was applied to assess mediation. The Genetic predictors for five risky behaviors were also obtained from the most up-to-date GWAS conducted by the UK Biobank (ever self-harmed: 117,733; ever attempted suicide: 4933; psychoactive substance abuse, alcohol use, and tobacco use: 463,010).MR analysis indicated a risky causal effect of age first had sexual intercourse (OR = 0.720, 95 % CI: 0.661-0.784, P = 2.45 × 10-14) and lifetime number of sexual partners (OR = 1.656, 95 % CI: 1.356-2.022, P = 7.46 × 10-7) on MDD. Mediation analysis showed the effects were mediated by tobacco use, with a proportion of 34.20 % on age first had sexual intercourse and 22.94 % on lifetime number of sexual partners separately.The overlap of participants in different traits and unclear gender.Robust genetic evidence indicated that premature sexual intercourse and more sexual partners were risks for MDD. Risky behaviors, especially the tobacco use, mediated this effect.