Why Your Big Sister Resents You

In a TikTok video that has been watched more than 6 million times, Kati Morton, a licensed marriage and family therapist in Santa Monica, California, lists signs that she says can be indicative of “eldest daughter syndrome.”

Among them: an intense feeling of familial responsibility, people-pleasing tendencies and resentment toward your siblings and parents.

On the social media platform X, a viral post asks: “are u happy or are u the oldest sibling and also a girl”?

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Firstborn daughters are having a moment in the spotlight, at least online, with memes and think pieces offering a sense of gratification to responsible, put-upon big sisters everywhere. But even mental health professionals like Morton — herself the youngest in her family — caution against putting too much stock in the psychology of sibling birth order, and the idea that it shapes personality or long-term outcomes.

“People will say, ‘It means everything!’ Other people will say, ‘There’s no proof,’” she said, noting that eldest daughter syndrome (which isn’t an actual mental health diagnosis) may have as much to do with gender norms as it does with birth order. “Everybody’s seeking to understand themselves, and to feel understood. And this is just another page in that book.”

What the Research Says About Birth Order

The stereotypes are familiar to many of us: Firstborn children are reliable and high-achieving; middle children are sociable and rebellious (and overlooked); and youngest children are charming and manipulative.

Studies have indeed found ties between a person’s role in the family lineup and various outcomes, including educational attainment and IQ (though those scores are not necessarily reliable measures of intelligence), financial risk tolerance and even participation in dangerous sports. But many studies have focused on a single point in time, cautioned Rodica Damian, a social-personality psychologist at the University of Houston. That means older siblings may have appeared more responsible or even more intelligent simply because they were more mature than their siblings, she said, adding that the sample sizes in most birth order studies have also been relatively small.

In larger analyses, the link between birth order and personality traits appears much weaker. A 2015 study looking at more than 20,000 people in Germany, the United Kingdom and the United States found no link between birth order and personality characteristics — though the researchers did find evidence that older children have a slight advantage in IQ. (So, eldest daughters, take your bragging rights where you can get them.)

Damian worked on a different large-scale study, also published in 2015, that included more than 370,000 high schoolers in the United States. It found slight differences in personality and intelligence, but the differences were so small, she said, that they were essentially meaningless. Damian did allow that cultural practices such as property or business inheritance (which may go to the first born) might affect how birth order influences family dynamics and sibling roles.

Still, there is no convincing some siblings who insist their birth order has predestined their role in the family.

After her study published, Damian appeared on a call-in radio show. The lines flooded with listeners who were delighted to tell her how skewed her findings were.

“Somebody would say: ‘You’re wrong! I’m a firstborn and I’m more conscientious than my siblings!’ And then someone else would call in and say, ‘You’re wrong, I’m a later-born and I’m more conscientious than my siblings!” she said.

What Personal Experience Says

Sara Stanizai, a licensed marriage and family therapist in Long Beach, California, runs a virtual group with weekly meet-ups, where participants reflect on how they believe their birth order has affected them and how it may be continuing to shape their romantic lives, friendships and careers.

The program was inspired by Stanizai’s experience as an eldest daughter in an Afghan-American family, where she felt “parentified” and “overly responsible” for her siblings — in part because she was older, and in part because she was a girl.

While Stanizai acknowledged that the research around birth order is mixed, she finds it useful for many of her clients to reflect on their birth order and how they believe it shaped their family life — particularly if they felt hemmed in or saddled by certain expectations.

Her therapy groups spend time reflecting on questions like: How does my family see me? How do I see myself? Can we talk about any discrepancies in our viewpoints, and how they shape family dynamics? For instance, an older sibling might point out that he or she is often the one to plan family vacations. A younger sibling might point out that he or she often feels pressured into going along with whatever the rest of the group wants.

Whether or not there is evidence that birth order determines personality traits is almost beside the point, experts acknowledged.

“I think people are just looking for meaning and self-understanding,” Stanizai said. “Horoscopes, birth order, attachment styles” are just a few examples, she said. “People are just looking for a set of code words and ways of describing their experiences.”

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