When people spend most of their time around someone, they tend to adopt each other's little quirks. Best friends speak their own language, roommates start doing chores the same way, and so on. Underneath it all, though, our personalities remain the same — right? According to recent research, some personality traits are actually contagious, so the stuff that makes you yourself might be more flexible than previously thought.
In a study published in the Journal of Personality and Social Psychology, researchers from Michigan State University followed two preschool classrooms (53 students total) over the course of a school year. During that time, they kept an eye children's developing personalities and which of their peers they chose to play with. Three- and four-year-olds can't take the same kind of complex personality inventories as adults, so researchers focused on three dimensions of temperament: positive emotionality (PE), negative emotionality (NE), and effortful control (EC). PE has been described as a "superstructure" of positive emotional functioning, while NE is a tendency toward emotions like frustration, anxiety, or sadness. EC basically describes a child's ability to self-regulate.
It's thought that these traits develop into more solid personality traits as you grow up; for example, research has shown a correlation between PE and extroversion later in life.
Unsurprisingly, these traits influence how you form relationships with other people as a child. In the study, when researchers compared these characteristics with children's choices in playmates, they found that kids were more likely to form friendships with classmates who had similar temperaments. Here's the interesting part: The study authors wrote that once that friendship was formed, "children’s level of PE and EC changed such that they became more similar to their playmates in levels of these traits."
In other words, preschoolers became more similar the longer they played together. If you've ever gone through a fitness phase when you dated a wannabe bodybuilder, it's a familiar situation.
It's important to note that only PE and EC were communicable, so to speak; NE tended to be stable. However, researchers did find that kids with higher levels of NE were less likely to form social relationships over time.
Although you're presumably long past the preschool stage, the study's findings are relevant even for adults. In the past, research has shown that most individual differences remain stable over the years, but the Michigan State study supports a few very recent studies indicating it's possible to change your personality if you really want — just don't expect instantaneous results.
The moral of the story? Personalities are more malleable than you might think, and your friends could wield a strong (not to mention positive) influence. Choose your squad wisely.