The well-known saying that "opposites attract" may not always be accurate, according to a recent report from the University of Colorado Boulder.
A group of researchers reviewed previous studies and their own original data analysis of more than 130 traits spanning millions of couples, as far back as the year 1903.
They found that partners were more likely to be similar, sharing between 82% to 89% of the traits analyzed, according to a news release from the university.
The group also found that individuals were likely to partner with those who were different from them for only 3% of the analyzed traits.
The study was published last month in the journal Nature Human Behavior.
"Our findings demonstrate that birds of a feather are indeed more likely to flock together," first author Tanya Horwitz, a doctoral candidate in the Department of Psychology and Neuroscience and the Institute for Behavioral Genetics, said in the release.
Political and religious attitudes, level of education and substance use were traits that showed the highest correlations, the study found.
Birth year was the most shared trait.
"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," Horwitz said in the release.
Heavy drinkers, smokers and nondrinkers had a strong tendency to partner up with those who had similar substance use habits.
Other parallels were seen for weight and height, personality traits and medical conditions.
Even the number of sex partners a person had and whether the individuals were breast-fed as infants showed some correlation, according to the report.
Extroverts, however, did not follow a typical pattern when it came to choosing a partner who was either an introvert or an extrovert.
"People have all these theories that extroverts like introverts or extroverts like other extroverts, but the fact of the matter is that it’s about like flipping a coin — extroverts are similarly likely to end up with extroverts as with introverts," Horwitz said in the news release.
Further research is needed to explore correlations that could have an impact on the genetics of future generations, the researchers stated.
If tall people are more likely to couple up with tall people and short people are more likely to partner up with short people and both groups have children, there could be more people at both height extremes in the next generation, Horwitz said in the release.
The same goes for other characteristics, including medical and psychiatric traits.
There are also potential social implications, the researchers noted. If individuals are likely to partner up with those of similar educational backgrounds, this could eventually widen the socioeconomic divide.
"We’re hoping people can use this data to do their own analyses and learn more about how and why people end up in the relationships they do," Horwitz said.
These correlations may occur for several reasons, the authors said. Some grew up in the same area, were attracted to those similar to them, or became more similar over time, according to the report.
The study did not look at same-sex relationships because the patterns may differ significantly, the release noted.
The researchers are now exploring those separately.
Some psychologists and relationship experts, who were not involved with the study, weighed in on the study’s findings in comments to Fox News Digital.
Dr. Jayme Albin, PhD, a cognitive behavioral psychologist in New York City who also counsels those dealing with relationship issues, said she was not surprised to see couples highly correlated for personality traits such as political and religious attitudes, level of education, certain measures of IQ, and attitudes about sex and substance use.
"These are important factors in how people lead their lives and the decisions they make when it comes to spending money and raising children, which are two topics that could heavily interfere with a marriage," she said.
"I think many couples when dating should pay closer attention to these traits — possibly more so than other elements, such as physical attraction or hobbies," added Albin.
Dr. Christine MacInnis, a licensed family therapist in Torrance, California, told Fox News Digital that the "opposites attract" notion may not prove to be a foundation for a longstanding relationship.
"Most romantic relationships are built on shared experiences and commonalities that foster connection," she said. "Opposites may attract, but the connection will wane without being able to see something of ourselves in the other person."
Dr. Nancy Frye, PhD, a professor in psychology and chair of the department of behavioral sciences at Long Island University in Brookville, New York, noted that many studies have found that couples who are more similar tend to be happier.
"People tend to form relationships with people they interact with, who live in their neighborhood and frequent the stores and restaurants that they do," she told Fox News Digital.
"Part of this is connected to how people change over time. As a relationship progresses, people may expand their hobbies and interests to include those of their partner."
The social psychology expert also discussed why many people believe the saying that opposites attract.
"This perception is often due to the fact that people take on complementary roles in a relationship," she said. "One partner may be the one to organize the finances, while the other partner may be the one to dream up things to spend money on."
She added, "But if a pool of people are compared to strangers and the people with whom they’re in a relationship, they are much more similar to their relationship partners than to the strangers."