For a 2016 study published in Brain Imaging and Behavior, researchers recruited 303 participants, a mix of only children and people with siblings. After taking an intelligence test, a personality test that measured the Big Five personality traits, and the Torrance Tests of Creative Thinking, the participants underwent MRI scans so researchers could get images of their brains. The tests showed that while both groups had similar intelligence levels, only children tended to exhibit more flexibility in their thinking—a key indicator of creativity—and less agreeableness. Previous research has found similar behavioral results, but where this study took it one further was by finding neurological differences between the two groups.

The MRI scans revealed a distinct difference in the volume of gray matter in the brains of only children and those of people with siblings. Only children showed more gray matter in a portion of their brain's parietal lobes called the supramarginal gyrus, which controls their language control and processing. The researchers correlated that difference with the only children's flexibility—i.e., their creative thinking.

On the flip side, only children demonstrated a lower volume of their brain's medial prefrontal cortex—the part of your brain that controls personality and social behaviors. Researchers linked this finding to those lower agreeableness scores.

This isn't to say only children are born with different brains. Especially when it comes to creativity, it's all in their upbringing. Researchers suggest that parents of only children may spend more time and effort on them, therefore setting more concrete expectations and giving them more opportunities than they would otherwise. "Additionally," the study explains, "only children might have more opportunities for independent activity, and independence is strongly related to creative thinking."

As for the low agreeableness scores, that extra time with parents could be a drawback. As Science Alert explains, "only children's lesser agreeableness could result from excessive attention from family members, less exposure to external social groups, and more focus on solitary activities while growing up." Every experience you have shapes your brain, so it makes sense that two groups with different upbringings would show differences in their brain structure. But now we have proof. Only children aren't just raised differently—their brains are different, too.

The study's co-author, Regina Miranda of Hunter College, explains to PsyPost that previous research focused on "understanding and changing negative cognitions (including cognitions about the future)," but their study aimed to uncover why even positive fantasies about the future can leave us bumming. It's all in a word: rumination—basically, thinking about something over and over and over.

How can thinking about something be bad? After all, you were only looking at hiking gear for Machu Picchu. That's harmless, right? According to the study, yes and no. Miranda and her colleagues write that rumination can take two forms: reflection, referring to goal-oriented reflection that could lead to real solutions; and brooding, which involves just dwelling on a bad mood.

While we generally consider fantasizing about the future to be in that former camp, the study, which surveyed 261 undergraduate students about just this thing, found that that's not necessarily the case. If you do it once in a blue moon, you're probably fine. However, "when people indulge in positive fantasies about the future more consistently, or when people's fantasies increase," they start brooding. Stewing in that bad mood can make it more difficult to be optimistic about the future, and you brood even more. A real catch-22.

Ok, so even if you do research flight deals to Peru on a regular basis, why would that necessarily lead to brooding? Miranda and her colleagues laid out two ways this could happen. First, if you fantasize about traveling to South America, but you don't actively commit to things like saving money, securing the days off from work, or getting your body hiking-ready, this failure to reach your goal could lead to depressive symptoms.

Or, in the more abstract, maybe you never actually believed that a trip to Peru was possible in the first place. Now, that's a bummer. Miranda acknowledges that more research needs to be done, however, and even notes that a third variable may be involved.