The power of animal magnetism has brought countless couples together, but when it comes to who we fall for, scientists say there’s little truth in the old adage that opposites attract. A study on romantic relationships found that for more than 80% of traits analysed – from political views to drug taking and the age at which people first had sex – partners were often remarkably similar. Tanya Horwitz, a PhD student at the University of Colorado Boulder, US, and the first author of the study published in the journal Nature Human Behaviour, said: “Birds of a feather are indeed more likely to flock together.” According to the research, between 82% and 89% of traits examined were similar among partners, with only 3% ranking as substantially different. For the study, scientists reviewed previous research on how similar or dissimilar couples tended to be, work that covered 22 traits across nearly 200 papers involving millions of male-female partnerships dating back to 1903. The group then followed up with a fresh analysis of 133 traits in nearly 80,000 opposite-sex couples enrolled in the UK Biobank project. Because behaviour may differ for same-sex couples, the scientists are investigating these relationships separately. In both pieces of work, couples largely matched across a range of traits including political and religious views, levels of education and some measures of IQ. Heavy smokers, heavy drinkers and teetotallers all tended to partner up with people who shared their habits. But couples did not match on every front. Height, weight, medical problems and personality traits all varied among couples. Extroverts, for example, were no more likely to partner up with other extroverts than introverts. Horwitz said: “The fact of the matter is that it’s like flipping a coin.” Couples were likely to share a similar birth year, and show similarities in terms of less well-studied traits such as how many sexual partners they had had and whether they were breastfed as a baby. When opposites did appear to attract, the associations were often weak and uncertain. This was seen in early risers pairing with night owls, left-handed people with right-handed, and those who have a tendency to worry with those who do not. “Even in situations where we feel like we have a choice about our relationships, there may be mechanisms happening behind the scenes of which we aren’t fully aware,” Horwitz said. The work builds on previous studies that suggest romantic partners often share core beliefs, values and hobbies, as may be expected. Relationships based on common ground can arise when people grow up in the same area, socialise with a narrow group of friends, or grow more similar the longer they spend together. Coupling along common lines could have future consequences, the researchers note. For example, if taller people pair up with other taller people, and shorter people with other shorter people, the coming generations could have more individuals at the extremes of the population’s height distribution. The same applies to social habits and other traits. Some studies suggest that people increasingly pair up along educational backgrounds, raising concerns of a widening socioeconomic divide.