What is eldest daughter syndrome? Millennials have long joked about it, and now science is backing them up

'Eldest daughter syndrome’ has long been an online phenomenon, with millennials talking about the added responsibility they felt growing up as the oldest siblings, but new research has backed up their claims and now science is on their side.

Being a sibling in a family of any size comes with unique challenges. For one, there's the ubiquitous challenge of sibling rivalry, though science has shown that kids who fight with their siblings will fare better in life, and surprising research has also shown that teenagers struggle more with their mental health when they have siblings. 

But what about being the eldest sibling specifically? There's long been sympathy for the middle child, and the baby of the family always receives special treatment, but the eldest sibling feels largely...forgotten. 

That's what millennials have long been saying online, anyway, especially as they reveal what they wish their parents had done differently while they were growing up. 'Eldest daughter syndrome' is the name given to the concept, explaining how eldest daughters tend to mature quicker than their siblings as they help out parents with bringing up the younger members of their family. But the 'diagnosis' is not one psychiatrists, therapists or counsellors are giving in sessions, it's purely been an online phenomenon - that is, until now. 

Researchers at the University of California, Los Angeles, after a 15-year-long study, have found evidence suggesting that first-born daughters not only mature quicker mentally, but also do so physically, when their mothers have more children. The same effect was not seen in daughters who were not the first born or in sons who were the eldest child. 

So what's happening? Specifically, eldest daughters hit adrenal puberty, an early form of puberty, if their mothers experience a high level of prenatal stress during their subsequent pregnancies. The theory UCLA researcher and anthropologist Molly Fox has posed is that the 'early maturation may enable a first-born daughter to help her mother rear her other children successfully.' 

This early puberty is physically visible through changes like the growth of body hair, pimples and aspects of 'cognitive maturation,' but does not include the early onset of menstruation, the study explained. 

Jennifer Hahn-Holbrook, one of the co-authors of the study and an assistant professor of psychology at the University of California, said, “It gives mum a ‘helper-at-the-nest’ sooner, aiding the women in keeping the latter offspring alive in difficult environments." Great news for the mother and younger siblings, not so great for the first-born daughter.

Hahn-Holbrook added, “One reason that we didn’t find this effect in first-born children who are sons could be that male children help less often with direct childcare than female children do, so mothers have less of an adaptive incentive to speed their social pubertal development."

Eldest daughter syndrome largely manifests as a feeling of overwhelming responsibility for the child's family’s well-being. As one eldest daughter told The Huffington Post, "By the time my youngest brother was born when I was almost 11, I was overwhelmed with feelings of responsibility for his welfare. I used to sit by his crib and watch him sleep just to make sure he was safe.

“It wasn’t that I thought my mother wasn’t competent ― but more that I felt we were both responsible for the family by that point in my life. As if I was literally ‘other mum,’ rather than big sister.”

Still, not everything is against eldest daughters. One 2014 study found that eldest daughters are the most likely to succeed out of any sibling type, while a 2012 study found that those who are eldest-born are more likely to hold leadership roles.

In other family news, 'nearly all millennial parents think their approach to parenting is ‘better’ than previous generations’ - but still struggle with this relatable issue. The generation have also shared the biggest differences between their children's lives vs how they grew up - and they're surprisingly relatable, and they've also revealed their top parenting rules for 2024 and it proves they’re determined not to make the same mistakes their own parents did.