Introduction While the effect of birth order on psychosocial outcomes has been widely discussed in the literature, little research examining birth order effects on sexual health has been undertaken. This analysis identifies the associations between birth order and learning about sex, first sexual experiences and sexual risk behaviours.

Methods This analysis uses data from Natsal-3, a stratified probability sample survey of 15,162 men and women aged 16–74 in the UK. Bivariate logistic regression was conducted to identify crude odds ratios for the association between birth order and sexual health outcomes. Multivariate logistic regression was performed adjusting for socio-demographic factors and sibling number.

Results Middle-born and last-born men were less likely to have found it easy to speak to their parents about sex around age 14 (OR 0.59, p=0.003; OR 0.69, p=0.009) and to have learned about sex from their mothers (OR 0.64, p=0.014; OR 0.76, p=0.045). Last-born women were less likely to report a parental main source of sex education (OR 0.64, p=0.003). Being a last-born male was associated with decreased odds of having had 5+ lifetime heterosexual partners (OR 0.75) and reporting ever had heterosexual anal sex (OR 0.77).

Discussion These results provide the basis for further research on the association between birth order and learning about sex, and highlight later-born males in particular as being less likely to report parental involvement in sex education. Qualitative research is recommended in order to gain a broader understanding of the ways in which birth order effects manifest in learning about sex.