A major study has turned the age-old adage of 'opposites attract' on its head. 

Researchers at the University of Colorado Boulder analysed more than 130 traits across millions of couples using data spanning over a century. 

The analysed traits were wideranging and included birth year, political leanings, substance use habits, level of education, height, weight, medical conditions, and personality among others.

They found that individuals tended to partner with people different to themselves for only around 3% of traits.

For between 82% and 89% of the traits analysed, partners were more likely to be similar. 

"Our findings demonstrate that birds of a feather are indeed more likely to flock together," said first author Tanya Horwitz, a doctoral candidate in the Department of Psychology and Neuroscience and the Institute for Behavioral Genetics (IBG).

The study, published in the Nature Human Behaviour journal, analysed 22 traits across 199 studies dating back to 1903 that included millions of heterosexual parents, engaged couples, married couples and cohabitating pairs.

Their conclusion from this analysis was that there is "no compelling evidence" to support the notion that opposites attract on any of the traits studied.

An additional analysis looked at 133 traits using data from the UK Biobank that covered 80,000 heterosexual couples from the UK.

This analysis pointed to a handful of traits "in which there seemed to be a negative correlation, albeit small". These included a tendency to worry, hearing difficulties, and being a morning person or 'night owl'. 

Same-sex couples were not included in the research and will be studied separately by the researchers.

The most common shared trait was birth year but there were also "particularly high" correlations when it came to political and religious attitudes, level of education, certain measures of IQ.

Drinking and smoking habits were another area of common ground. While teetotallers tended to partner together, the same was true of heavy smokers and drinkers.

"These findings suggest that 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," said Horwitz.