A new article in Psychological Science sought to answer this question while counteracting biases in previous research. The problem is that in this area of research, we tend to rely on self-report and peer-report personality measures. And if people go into the study with expectations about what they think is true (i.e., are we birds or opposites?), that might bias how they respond. Instead, these researchers used behavioral measures of personality:
The first approach measured personality using a common type of digital footprint: Facebook Likes. Facebook users generate Likes by clicking a Like button on Facebook Pages related to products, famous people, books, etc. This feature allows users to express their preferences for a variety of content. It has been shown that Likes can be used to accurately assess people’s personality (Kosinski, Stillwell, & Graepel, 2013; Youyou, Kosinski, & Stillwell, 2015). For example, people who score high on Extraversion tend to Like “partying,” “dancing,” and celebrities.Using data from the myPersonality Facebook application, which allows users to take various personality measures (so all participants had at least some self-report personality results), they built models using like data and status update language data. These models were then applied to a sample of dyads (pairs of friends or romantic partners). They found that dyads tend to be similar, and this is especially true for members of a romantic couple:
The second approach measured personality using digital records of language use: Facebook status updates. Facebook users write status updates to share their thoughts, feelings, and life events with friends. Previous research has consistently found links between personality and language use (Hirsh & Peterson, 2009; Mehl, Gosling, & Pennebaker, 2006; Tausczik & Pennebaker, 2010). Extraverts, for example, tend to use more words describing positive emotions (e.g., “great,” “happy,” or “amazing”; H. A. Schwartz et al., 2013) than introverts do. Several studies have demonstrated accurate personality assessment based on people’s language use in social media (Farnadi et al., 2014; Sumner, Byers, Boochever, & Park, 2012), including Facebook status updates (Park et al., 2014; H. A. Schwartz et al., 2013).
Our findings provide evidence that romantic partners as well as friends are characterized by similar personalities. We measured personality traits relying on three different sources of data: traditional self-report questionnaires, digital records of behaviors and preferences, and language use. Relatively strong similarity was detected between romantic partners and between friends when we used Likes-based and language-based measures. By contrast, self-reports yielded only weak to negligible similarity. Across all three methods, stronger personality similarity was found for romantic couples than for friends.So based on this research, it seems we're birds.