Growing up as the oldest sibling, author Y.L. Wolfe often felt the lines between her role and her mother's role were blurred. "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," Wolfe, the oldest of four, told HuffPost. "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," she explained. "As if I was literally 'other mom,' rather than big sister."
In other words, Wolfe is deeply familiar with 'eldest daughter syndrome.' The internet is rife with thinkpieces about the plight of oldest daughters and tweets about how they need to unionize. Though 'eldest daughter syndrome' is a pop psychology term, a new study suggests there may be more science to the pseudo-syndrome than previously thought.
Research Findings
A University of California, Los Angeles-led research team found that, in certain instances, first-born daughters tend to mature earlier, enabling them to help their mother rear younger siblings. Specifically, the researchers found a correlation between early signs of adrenal puberty in first-born daughters and their mothers having experienced high levels of prenatal stress.
Why does age of adrenal puberty matter? Changes in skin and body hair happen during this phase, but so do changes in brain development. Adrenal puberty processes are believed to foster social and cognitive changes; basically, superficial physical changes correlate with emotional maturity.
"When times are tough and mothers are stressed in pregnancy, it's in the mother's adaptive best interest for her daughter to socially mature at a quicker pace," said Jennifer Hahn-Holbrook, one of the co-authors of the study and an assistant professor of psychology at the University of California, Merced. "It gives mom a 'helper-at-the-nest' sooner, aiding the women in keeping the latter offspring alive in difficult environments."
Key Distinctions
Notably, adrenal puberty does not include breast development or the onset of menstruation for girls. The study posits that girls become mentally mature enough to care for their younger siblings while not being physically capable of having their own children, which would naturally draw them away from their older daughter responsibilities. Older brothers are seemingly off the hook: The researchers did not find the same result in boys or daughters who were not first-born.
"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," Hahn-Holbrook explained. Plus, previous research suggests that female puberty timing is more malleable in response to early life experiences than males.
Study Methodology
The results of this study, published in the February issue of Psychoneuroendocrinology, were a long time coming: Researchers tracked the families for 15 years, from the pregnancy stage to the babies' teen years. Researchers recruited women from two obstetric clinics in Southern California during routine first trimester prenatal care visits. On average, the women were 30 years of age and pregnant with one child. It was their first pregnancy for roughly half of the participants. The women were nonsmoking and not using steroid medications, tobacco, alcohol or other recreational drugs during pregnancy. They were all over 18 years of age.
At five different stages of pregnancy, the women's stress, depression and anxiety levels were measured, and then measured cumulatively. Of the children born to these mothers, 48 percent were female and 52 percent were male. As the children aged, characteristics of adrenal and gonadal puberty were separately measured, including body hair, skin changes, growth spurts, breast development and menstruation in females, and voice changes and facial hair in males.
The study also measured childhood adversity to account for other factors known to correlate to early maturation, such as the death of a parent or divorce before age 5, absence of a father, and economic uncertainties at ages 7-9.
Broader Implications
Taking all that into account, it was the eldest girls who matured the fastest when their moms experienced high levels of prenatal stress. Other studies suggest there is some later-in-life payoff for highly responsible eldest girls: A 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.
"This is a first-of-its-kind finding and is fascinating to look at through an evolutionary lens," said Molly Fox, a UCLA anthropologist and one of the co-authors of the study.
The findings ring true for Wolfe. "I'm not at all surprised by what the study found," Wolfe said. "My story is slightly different — I went through true puberty, not just adrenal puberty, at 12, though I suspect I experienced an early cognitive maturation."
The study is also interesting because it adds to social scientists' growing understanding of fetal programming, which explores how stress and other emotional and environmental factors women experience during pregnancy affect their children long after birth. "One fascinating theory is that when you're still a fetus in your mother's womb, you get cues about what the world is going to be like, and your body can flexibly adjust the shape of your life-cycle to be optimally suited to those conditions you expect to encounter," Fox said.
Fox and her co-authors are excited their work is out there for the public to read, especially after following the families for so long. The fact that the findings were published just as a cultural conversation about eldest daughters broke out was a cherry on top, especially for Fox, a co-oldest daughter. "As co-oldest, I think it's a special role in any family because of the potential for closeness with my mother and capacity to help care for my younger siblings," she said.
