The findings, published in the journal Sex Education, suggest that birth order may play a significant role in how children learn about sex, especially for boys.
In the study involving 5,000 individuals, 48 per cent of first-born women and 37 per cent of first-born men reported discussing sex with a parent at age 14, as opposed to 40 per cent of middle-born women and 29 per cent of middle-born men.
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"In addition to seeing differences in sex education according to birth order, we also found clear differences between the sexes; across all birth order categories, men consistently reported lower parental involvement in sex education than women," said lead researcher Lotte Elton from London School of Hygiene and Tropical Medicine (LSHTM).
"Our findings suggest that the previously-reported difficulties men face in talking about sex with parents may be exacerbated if they are middle- or last-born," Elton added.
The researchers used data from the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3), the largest scientific studies of sexual health and lifestyles in Britain.
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The studies have been carried out every 10 years since 1990, and have involved interviews with more than 45,000 people to date.
Taking a sub-sample of Natsal-3 participants -- 5,000 individuals aged 17 to 29 who were either first-born, middle-born or last-born -- the team analysed responses to questions about the involvement of parents or siblings in sex education and early sexual experiences.
First-born children were more likely to report parental involvement in sex education compared with later-born children, according to the findings.
Middle-born and last-born men were less likely than first-born to report having found it easy to discuss sex with their parents growing up, the results showed.
Later-born men were also less likely to report learning about sex from their mothers, while last-born women were less likely than first-born women to report a parent as a main source of sex education.
Although there were differences by birth order in parental involvement in sex education, there did not appear to be an association between birth order and early sexual experiences, although middle-born men were at increased odds of being under 16 when they first had sex, the study said.