Patricia East started her career as a developmental psychologist at a women's clinic in California.
She saw lots of pregnant teens.
She tells NPR that a pattern started to emerge.
"The nurses and the doctors there would bring a teen back for her prenatal visit and they would say, 'Hey! Aren't you Maria's younger sister?'"she recalls.
"And the young woman would say, 'Yeah, I am!' And they would say to another patient, 'You know, haven't I seen you before?' And she would say, 'Yes, I was here for my older sister when she was pregnant.'"
As a good scientist does, East decided to verify her observation, and she followed pairs of sisters in a study.
The results were astounding: A woman whose older sister got pregnant was five times more likely to get pregnant than one who didn't.
And just as your parents told you, siblings can provide positive role models as well. Other research has found that support from older siblings can drive academic achievement. Academics call it "the sibling spillover effect," and there are at least three possible drivers — the older sibling helps with homework, the younger sibling imitates their work style, or the older sibling tips the young one to which classes and teachers to take.
This is all part of a growing body of knowledge around how our sibling relationships affect not only how we behaved as kids, but how we act as adults.
It's a new science. While psychologists have been investigating how family relationships affect our identities since the 19th century, it's mostly been focused on the mother-father-child triad, rather than relationships we have with siblings.
But if you think of a family as a tiny society, it makes sense that we would first learn to socialize through relationships with sisters and brothers.
In the formative 1995 book "Sibling Relationships Across The Life Span," psychologist Victor Cicirelli says that "the older sibling gains in social skills in interacting with the younger" and "the younger sibling gains cognitively by imitating the older." In this way, siblings are "agents of socialization." In other words, they provide the foundation of how we learn to deal with people.
As we've discussed before, siblings are constantly competing for their parents' attention, and that tendency toward competition is handed down through evolution.
The logic goes that the more a child of any species receives the energy of their parents, the more likely the child is to survive — thus the reason first children tend to be achievers, while second or third children jump at separate activities like athletics, music, or arts. Just like Darwin's finches, each sibling is trying to find his or her niche.
But as NPR reporter Alix Spiegel argues, it's good for them.
"[Siblings] learn from the friction between them ... as they fight for their parents' attention," she writes. "Mild conflict between brothers and sisters teaches them how to interact with peers, coworkers, and friends for the rest of their lives."
If siblings manage to get into adulthood without overwhelming resentment, then they tend to become BFFs. A survey of 7,730 Americans found that 30% of respondents would call a sibling in an emergency, and over 60% said their brother or sister was one of their best friends.
It lasts late into life: A Swedish study of people over 80 years old found that having closeness with friends or even children did little to increase total life satisfaction — but feeling close contact with a sibling did.
SEE ALSO: Firstborn Daughters Are The Most Ambitious Children, Research Finds