Always the first to lend a hand, to reassure, or to take charge? If you're the eldest sibling, these traits will surely resonate with you. For several years, the "eldest daughter syndrome" has been circulating on social media, but science is finally beginning to confirm some of its underlying mechanisms.
A role of "other mother" that takes shape from childhood
Many older girls report growing up with an early sense of responsibility. American writer Yael Wolfe explained to HuffPost that at age 11, she cared for her younger brother "like a second mother." It wasn't that her mother lacked skills, but she felt jointly responsible for the family's well-being.
These experiences are not isolated. Online, they resonate widely: humorous memes about "older sister emotional fatigue" or "mother hen bonuses" have been circulating for years. Behind the humor, however, lies a solid psychological reality: these girls mature emotionally and socially faster. And science is now providing concrete explanations.
Accelerated maturation linked to maternal stress
A study conducted by the University of California and published in Psychoneuroendocrinology followed more than 100 families for 15 years. Researchers measured mothers' anxiety, depression, and stress at different stages of their pregnancies. The result? Firstborn daughters of highly stressed mothers showed faster social and emotional development than their younger siblings.
This phenomenon is partly explained by "adrenal puberty," a phase of subtle hormonal and cognitive changes that precedes physical puberty. These adjustments prepare the brain for greater empathy, attentiveness, and a sense of responsibility. In other words, the bodies and minds of firstborn daughters seem naturally calibrated to become "nursery assistants" within their families. Jennifer Hahn-Holbrook, a psychologist and co-author of the study, explains: "When times are tough, it's advantageous for the mother if her eldest daughter matures faster so she can provide support. It's an adaptive mechanism."
A predominantly female phenomenon
Older boys do not follow the same pattern. According to researchers, they participate less in the direct care of younger children, which reduces the biological benefit of early maturation. Furthermore, female development is particularly sensitive to the family and social environment, especially maternal stress and early emotional interactions.
Thus, girls unconsciously adjust their development to meet their family's needs. This "parentification" role often brings advantages: autonomy, empathy, and leadership. It can also generate feelings of hyper-responsibility or guilt in adulthood.
When science meets collective intuition
The "eldest daughter syndrome" has no official medical recognition, but this study provides a scientific basis for what many intuitively felt. The eldest daughter is often the mediator, the protector, the one who ensures family cohesion. Science refers to this as fetal programming: the baby adapts its development to maternal stress signals, anticipating a demanding environment. A true evolutionary strategy: growing up quickly to support the family and facilitate its survival.
In short, the famous older sister isn't simply organized or responsible out of habit: her qualities are rooted in biology and psychological development. So, the next time your big sister takes charge or is overly protective, remember: it's not just a matter of character, it's practically written in her genes.
