• A new study explored how birth order, gender, and personality traits influence parental favoritism.
• It found that parents tend to prefer their oldest kids and also daughters.
• Parents may show preferential treatment toward eldest daughters, who they see as more responsible.

It's a fact of life that parents play favorites, but which children are favored more — and why — is an ongoing topic of research.

A recent study, published in the journal Psychological Bulletin in January, has shed some light on the matter, in hopes of pointing out how favoritism doesn't end well for anyone involved.

Which child parents are more likely to favor

The Brigham Young University researchers analyzed data from over 19,000 participants in 30 studies and 14 databases across North America and Western Europe.

They examined how birth order, gender, temperament, and personality played a role in favoritism and found that parents primarily tend to favor elder children, agreeable children, and daughters.

The data couldn't explain why this is the case, but the researchers have their theories.

Why parents favor eldest daughters

For example, daughters might be favored over sons because they're easier to parent and have more effortful control — the ability to sit still and ignore distractions.

Parents might also grant more autonomy, a measure of favoritism in the study, to older siblings because they are more "developmentally capable" than younger siblings.

Moreover, because eldest daughters may naturally take on more responsibilities and ease household burdens, "parents may naturally invest more in them," Annie Wright, a therapist practicing in California, told Business Insider.

She added that many cultures socialize girls to be "more emotionally expressive, empathetic, and communicative — traits that may make them more likable and easier for parents to bond with."

At the same time, the golden child can also wear a heavy crown.

The consequences of being the favorite

Wright is the eldest of six and identifies with eldest-daughter syndrome.

She told BI that there's a "shadow side" to being the favorite, such as people-pleasing and feeling relentless pressure to perform. Over time, it can lead to resentment and tension between siblings, Wright said.

Moreover, eldest daughters like Wright — who was given more chores growing up — can feel overly responsible for others. This mentality can eventually put them in a position as the caretakers in their families, a role that some lament because they feel like it holds them back.

Despite the societal messaging that women should make their own decisions, "there's still pressure for girls to care for the emotional needs of people in their immediate surroundings," Dr. Michelle Janning, a professor of sociology at Whitman College, told BI.

She added that these two expectations — to be great caretakers and to reach their ambitions — aren't mutually exclusive. "They are both in existence, but what's not happening maybe is how to manage both of them," she said.

In practice, it can lead to burnout and workaholism. Wright, for example, said she used to work 80-hour weeks while missing time spent with her daughter.

Wright added that eldest daughters, as a result, can also struggle with self-expression and independence later in life.

That said, life isn't too rosy for the less-favored kids, either.

Parents can make small adjustments

The study's authors believe that less-favored kids, like more defiant younger sons, are "at greater risk for maladaptive outcomes" because they receive less support from their parents.

In general, playing favorites isn't good for anyone involved and "negative outcomes happen for all siblings in the family," said Dr. Alexander C. Jensen, an associate professor at BYU and the study's lead researcher,

Parents can do small things to improve the dynamic, Wright said. They can ask themselves if they impose similar rules on their kids or if they provide more emotional attention to one child. And if they sense disparities, it's time to "level things out."

• I have two daughters, ages 4.5 and 1.5 years old.
• I wanted one kid, but my husband felt we should give our oldest a sibling.
• I want them to be friends and maybe I'm projecting the relationship I wanted to have with my brother.

"Can you hold your sister's hand?" I ask my oldest as we walk through the crowded airport.

I decided to fly solo with both kids for a long weekend. In doing so, though, I also put a lot of trust in my 4.5 and 1.5-year-old daughters to get through the airport as a unified team.

"Her hand is sweaty," Maeve said, shaking her little sister off.

"Please," I try again. "She's your little sister and best friend." It's hard to mask the pleading in my eyes as I roll two carry-on suitcases and have absolutely zero free hands.

"Ugh, fine," Maeve said. Gemma smiles when her big sister grabs her hand again. And I smile, too.

My husband and I disagreed on the number of kids we should have

My husband, an only child, had felt a bit lonely growing up, whereas I'd had a little brother and knew friendship wasn't guaranteed. While I was happy with one, he wanted our daughter to have a sibling. He figured they'd be best friends.

I told him we didn't know if that was the case. They could very well hate each other.

My brother and I are only 18 months apart. I remember being inseparable as little kids — we looked out for each other, spent hours riding bikes together, and would put on elaborate plays for our parents. Today, though? We have a fraught relationship. It was a divide that began when I was in middle school and has continued well into adulthood, thanks to a combination of differing personalities and the competitive atmosphere my parents unintentionally fostered.

I want my kids to be friends

Now that I have two daughters who are three years apart, I've made it a goal that they not only have a strong relationship but are also friends — maybe even best friends.

"You and your brother are best friends?" I once asked a coworker when I was pregnant with my second daughter. She'd told me she was traveling with her brother — something I could never imagine doing. "How did that happen?"

She said that her mom just kept telling them they were best friends, and so that's what they became.

Could it really be that simple? Almost immediately, I started telling my oldest that her new little sister would be her best friend.

As it turned out, it's obviously not that simple. When Gemma was born, Maeve was (understandably) upset that this screaming potato with eyes had come in and taken her mom's attention. It wasn't until the six-month mark that Maeve seemed to finally accept that Gemma wasn't going anywhere. And then, at around eight months, Maeve made Gemma laugh. Which seemed to give Maeve just enough satisfaction to try to make her laugh again.

Their relationship has ups and downs

Almost two years into being siblings, I watch their relationship ebb and flow — sometimes hugging and giggling, other times crying and screaming. The book "Siblings Without Rivalry" told me to set expectations low — they don't need to be friends, the goal should be for them to have a good relationship.

Still, I want them to be friends. I know that I'm projecting the relationship I wish I'd had with my brother onto them. But I can't help it — I don't want them to grow up and not be able to rely on each other. While I know I can't force them to willingly spend time together (they may very well end up like my brother and I), there are some things I can control.

So, for now, I try to plan activities they both like to do together. I occasionally remind them they're built-in best friends. And when something good happens for one of them (like my daughter's holiday recital), I make sure her sister cheers her on from the crowd.

Soon enough, I won't be able to dictate how they spend their time, and it will be up to them to decide their future relationship. Selfishly, I hope that relationship is friendly, and I have to hold on to that.