Why You Fight With Your Siblings Over Thanksgiving, According To Science

Woody Harrelson didn’t know that Liam and Chris Hemsworth were brothers, but now he does, and so can you! CREDIT: JOHN SHEARER/INVISION/AP
Woody Harrelson didn’t know that Liam and Chris Hemsworth were brothers, but now he does, and so can you! CREDIT: JOHN SHEARER/INVISION/AP

Thanksgiving is about tradition. Eating traditional food, watching the traditional Macy’s Day Parade, the traditional listening to the college freshman in the family point out that the Pilgrims and the Native Americans didn’t necessarily just sit down and get along as the lore would suggest.

To paraphrase one of our most famous phrases: you get older, but your childhood home seems to stay the same age. And there is something about the return to that house that can make you feel like you, too, are going back to some earlier age: the age that you were when you and your siblings last lived together under one roof. In anticipation of a long weekend back in our twin beds, it’s worth looking into the way birth order continues to shape your relationships, even long after everyone has moved away from home. Is it possible to escape that return-to-form feeling that so often accompanies family events?

Let’s ask scientists!

“It’s actually quite an interesting question because, just anecdotally, people often say that they go back for family holidays and find themselves falling back into the old patterns of relating to siblings,” said Frank Sulloway, an adjunct professor of psychology at UC Berkeley and author of Born to Rebel: Birth Order, Family Dynamics, and Creative Lives when we spoke by phone. “This sometimes comes as a surprise, that it happens so easily. At the scientific level, we have absolutely no direct evidence to say that this is the case.”

Oh. Okay, then. Thanks everyone for coming by, have a great hol —

“However, there’s a fair amount of indirect evidence that suggests this or something like it, is the case.” Sulloway cited his research from Why Siblings Are Like Darwin’s Finches: Birth Order, Sibling Competition, and Adaptive Divergence Within the Family: “When you have siblings rate themselves and simultaneously rate another sibling, the birth order effects that you get, in terms of personality, are substantially larger than when you just ask someone to rate themselves on personality scales.”


Say you were to rate, on a scale of one to five, how much the statement “I see myself as a thorough worker” applied to you. Sulloway’s research shows that you would actually give yourself a different number if you were to rate yourself and a sibling, as opposed to just rating yourself. “It’s not only possible, but quite plausible, that some of the difference in those ratings is a product of, essentially, sibling stereotypes,” he said, and “people fall into those stereotypes under circumstances that remind them of those stereotypes.” So if the stereotype of a firstborn is that he or she “is the surrogate parent, goody-goody, conscientious, responsible one,” the firstborn is all the more likely to behave that way when back in a family setting.

But the important thing to remember about birth order is that it, in and of itself, is not a cause of anything; birth order is a proxy, or “indirect variable, that is a marker of things that are truly having a causal effect,” said Sulloway. In the case of that firstborn, “it’s perfectly reasonable to suspect or believe that some of the difference is actually reflecting a real difference in the following sense: firstborns do tend to be surrogate parents, to be the sort of responsible head of the family, as far as siblings are concerned. For example, if the parents need a babysitter, they don’t ask the younger sibling to babysit the older sibling. If they send you out to buy a loaf of bread, they give the money to the older sibling.”

“The biological fact that you’re first, second, or third, it doesn’t have any causal influence. What’s causal is, if you are first, you’re bigger, older, stronger, and you have certain privileges you don’t have when you’re younger,” he said.

Time for more fun with stereotypes: “If firstborns act more, within the family context, as the surrogate parent, not only are they viewed stereotypically as more responsible and harder working — what we’d call conscientious — but they actually are behaving that way with reference to their siblings and the family environment. And also within the family, typically, younger siblings are rated as being more agreeable, which basically means, more cooperative, kindly, less aggressive, less bossy.” If you’re an older sibling, with size and power on your side, “if you want something, in theory you can just take it, and sometimes older siblings just do. And if you’re younger, you have to be more careful about how you behave with a bigger person. You tend to be cooperative, like, ‘I’ll give you this if you give me that.’”


But here’s the funny thing about those differences — and, I think, why family gatherings seem to exacerbate these variances in personality: these differences are much larger within the family context than they are outside of it. “If you’re used to getting away with bossing your siblings around, and that’s the way things are done in early childhood, and you grow up and do that to your friends, you’re not going to have many friends.” As a result, there’s “a muting of these differences” when you’re with friends, coworkers, and so on.

Sulloway has a very Hunger Games-y explanation for this phenomenon. “I like to think of it as, in childhood we develop a little Darwinian toolkit of strategies for dealing with our siblings and basically getting out of childhood alive.”

I suggest this is a harsh way of looking at things. Sulloway concedes that it does “sound harsh” but reminds me that “before 1800, half of all children died in childhood. And firstborns all around the world [were] more likely to survive than your younger siblings are… And studies show that laterborns are shorter and weigh less than their older siblings. So in a Darwinian world, tiny differences make a big difference in survival. So there is a survival benefit to being a firstborn, and the thing that younger siblings are doing are trying to impress their parents to send more resources their way. And since you can’t do that by being aggressive, you do it by trying to be agreeable and affectionate, which are all laterborn traits. That’s why I call it a Darwinian toolkit: it really is. But we don’t go around in adulthood taking stuff out of that toolkit all the time. If you did all the stuff to your friends that you do to siblings, you’d have no friends. So it makes sense that there’s a muting of effects but a continuity of effects.”

This is, perhaps, why Friendsgiving is often a less dramatic affair than actual Thanksgiving. Because “to go back to Thanksgiving, you go home, and you still have the toolkit! To the extent that another sibling slips back into the role that they had within the family, you’re going to react as you did from the perspective of your own role in the family… The common perception that siblings at holidays revert back into roles makes sense.”

Sulloway doesn’t think that the particular pressures of a holiday like Thanksgiving, with its emphasis on family, history and routine, is what causes this script to persist. “We could invent a totally new holiday, Bunny Day, and force every family to go back home for Bunny Day.” (He clarifies that this fictional holiday is separate from Easter. It’s a non-denominational fake holiday.) “There would be no tradition. But if siblings sat down and had a meal and so forth, they would still be likely, one would guess, to slip back into the same roles that they slip into at Thanksgiving. I think it’s just getting them all together in the same context. And particularly, I think it’s quite important that these involve the parents as well. Because it’s competition for what psychologists call parental investment, and parental investment is what gets you out of childhood alive.”


Ah, this be the verse. “So firstborns tend to have an advantage because they start out getting a bigger share of everything, because there are fewer people to share it with… Because of that tendency, middle children get the short end of the stick. Firstborns have a time when no one else is there, and lastborns have a time when no one else is there, because the older ones have moved away, so you get a larger proportion of resources [then]. And the middle child Is always competing with somebody else… So we know that middle children get the least amount of parental investment. They get the least amount of parental care hours [but] on the more positive side, middle and later children in general are somewhat more peer-oriented.” Sulloway brought up a study by Catherine Salmon in which she asked people, “how would you react if you witnessed a horrible car accident, you were upset, and needed to talk to somebody?” She found that firstborns and lastborns tend to say they’d call their parents, while middleborns say they’d call their best friend. (No word on only children! Sorry.) As Sulloway said, “It makes sense that if you’re getting less parental investment, you’ll get that investment elsewhere. So you make up for these disparities by just having stronger peer alliances.”

Richard Zweigenhaft, a psychology professor at Guilford College, also thought parents were a powerful part of the equation. He does think “there are some predictable differences between people who have different birth orders. The data is pretty compelling that firstborns tend to differ from laterborns in certain ways and vice versa.” Though he did want to qualify that “I wouldn’t be too quick to assume it would lead to specific issues at Thanksgiving,” because, as a man of science, he wasn’t willing to bend his data to my news peg. He offered that the question might not even be so much about birth order as it is about “how adults revert to, or regress to, earlier developmental stages and become, say, teenagers in the face of their parents, even if they’re 40 or 50 years old. There’s something to that. I think that does happen. And I, therefore, think the birth order patterns that were there growing up are likely to still be a part of the relationship.”

Zweigenhaft, too, offered some fun stereotypes you can bust out at the dinner table to praise or enrage your siblings, depending on your birth order and their sense(s) of humor: “Firstborns tend to be high achievers, they tend to be more traditional. Laterborns tend to be more rebellious, more likely to challenge the status quo.” He did not mention middle children, because, like a parent, he neglected to provide that group with an equal amount of attention. Rough life, middle children.

So what does this all mean for Thursday evening festivities? Are we doomed to play out the same rigid roles? Can we control our nature, no matter how aware we are of the ways in which it controls us?

“I think it’s better to view the family situation and these roles, they’re somewhat like a chess game,” Sulloway said. “These roles are not fixed in stone. They’re fluid, in the sense that siblings are doing all sort of sneaky things to get what they want from siblings and parents.” It’s all really quite moving. “So probably lots of times when middle and lastborns become super conscientious, [that’s] because that will produce brownie points.” It’s a way to address how “the game is sort of rigged.” Birth order effects are, in one way, relatively small, but then again, so is nearly every other influencing factor: on the environmental side (so, excluding genetics from the equation), the only larger systematic source of personality differences than birth order is gender. Birth order effects “explain maybe one-to-two-percent of behavior, but that is pretty large. That’s the equivalent of increasing the odds of you doing something by about 50 percent.”

Zweigenhaft’s guidance on the matter is all very accept-the-things-I-cannot-change: he and his older sister “have grown up and matured, [so] some things have fallen into place just because we’re different people. In some ways, I accept that she’s different than I am, and I don’t necessarily try to fight the fact that she’s difference.” He added that seismic life events — in his case, living through the death of both parents — forced the two “to work together to go through all of that stuff, [and] in some ways we became equals in ways that we weren’t before that.”

I asked if specific issues might be more likely to flare up old-time resentments; circumstances that parallel adolescent experience, like duking it out for the keys to the car. “I think you may be onto one of the things that does stick with you, no matter how old you get, and that is competition among siblings,” he said. “If you go back to what I was talking about before, parental attention is important. Young kids want their parents’ attention. Firstborns get a monopoly on it, and laterborns get a lot, too. That competition which can be intense. It varies from family to family. All those things [like who gets the car] probably don’t matter that much. But it has a big psychological impact.”

“But in terms of predicting what’s going to happen Thursday afternoon, that gets harder,” Zweigenhaft went on. “I don’t know that it’s always going to be the same dynamic” as siblings experienced as children, “but the original dynamic is still in there,” no matter how old you get. He also mentioned another reason Friendsgiving might be a less stressful event for some: “If we were to give a good personality test to you and your siblings, it might, the data show that you’re not likely to be real similar to your siblings on measures of personality. Correlation, the fancy statistic that’s used, is real low for siblings… You might have more in common with other firstborns, middleborns or lastborns in terms of personality than you do with your siblings. You’re clearly going to share a lot with siblings in terms of genetic stuff — you might look a lot like your siblings — but in terms of personality, birth order may be as good as or better a predictor. You might be more similar to people of the same birth order as you than to your other siblings.”