Bruno Pugh
People often joke about the oldest daughter being the “third parent,” but there’s more to that idea than meets the eye. Researchers are beginning to notice patterns that explain why the role of firstborn daughters feels heavier and different.
This piece is not about family myths; it’s about how real experiences shape behavior. Interested in how science unpacks this everyday story? Stick around, the answers are closer than you think.
Growing Into Responsibility Early
According to a UCLA-led study published in Psychoneuroendocrinology (February issue), first-born daughters showed earlier signs of adrenal puberty when their mothers experienced high levels of prenatal stress.
Adrenal puberty—more precisely called adrenarche—is the early phase of puberty when the adrenal glands start producing hormones like DHEA and androgens. The process involves shifts in skin, hair, and, importantly, brain development.
These changes are linked to quicker emotional and social maturity, giving eldest daughters the capacity to help with younger siblings sooner. This pattern suggests that eldest girls may naturally assume caregiving roles earlier than expected, influenced not by choice but by biology. The study tells us one thing: family roles are shaped before children even have a say.
Why Mothers Benefit From It
Jennifer Hahn-Holbrook, assistant professor of psychology at UC Merced and co-author of the study, explained that early social maturity benefits stressed mothers. “It gives mom a ‘helper-at-the-nest’ sooner, aiding the women in keeping the latter offspring alive in difficult environments,” she noted. The finding reflects an adaptive advantage in families facing hardship.
In other words, daughters develop faster socially, so they can help their mothers manage daily responsibilities. The science reframes what many have casually called “eldest daughter syndrome” as part of an evolutionary stress response.
What Happens To Brothers?
Puberty timing in boys is less flexible in response to early life conditions, while girls’ development shows greater sensitivity to environmental factors.
This distinction explains why many eldest daughters recall playing a “second parent” role, while brothers of the same birth order may not have had a similar experience.
Long-Term Outcomes For Eldest Girls
While early responsibility may feel like a burden, previous research shows positive payoffs. A 2014 study found that eldest daughters are more likely to succeed compared to other siblings, and a 2012 study revealed they often hold leadership roles. Responsibility placed on them early may translate into resilience, achievement, and confidence in adulthood.
An Evolutionary Perspective
Molly Fox, UCLA anthropologist and co-author of the study, described the findings as “a first-of-its-kind” and fascinating through an evolutionary lens. The concept of “fetal programming” explains how stress during pregnancy can influence a child’s lifelong development. Signals in the womb may guide the child to adapt to the expected world ahead.
This insight deepens the cultural conversation about the experiences of the eldest daughter. What feels like a social expectation may, in fact, have biological roots that reach back to pregnancy.
Why This Study Matters
The UCLA researchers followed families for 15 years, and this made their data especially strong. Their findings provide evidence for what eldest daughters and their families have long described: a feeling of blurred lines between being a sibling and being a caretaker. Though “eldest daughter syndrome” isn’t a clinical diagnosis, science is showing it’s more than just internet banter.
The results affirm the voices of countless eldest daughters.
If you’ve ever felt like the “other mom” in your family, the research now suggests there’s real biology behind that experience.