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If you've ever overheard elderly couples bickering about politics, despite decades of marriage, new research might help explain why.
A just-published study that's tracked more than 170 Kiwi couples over time has busted the myth that we gradually grow more alike to our partners – and instead suggests we simply change in our own way.
One of the biggest questions facing researchers in relationship science has been whether couples ultimately become similar to one another.
In a widely-publicised paper last year, for instance, researchers found no evidence to back a long-floated theory that couples came to physically resemble each other more over time – even though we might be initially drawn to people with similar-looking features.
A more plausible possibility was that we just ended up sharing world views more, or that our personal wellbeing, for better or worse, rubbed off on each other.
"For example, we know that couples tend to pick up diet and exercise habits from one another," Victoria University of Wellington senior lecturer Dr Matt Hammond explained.
"Some couples go to the gym together and perhaps then reward themselves with a fast-food meal together.
"Researchers know that peoples' traits, values, and beliefs change across time, but do they change in the same ways that their romantic partner is changing?"
Hammond said the psychological side to the question was particularly interesting, and carried implications for understanding the development of mental health.
"One theory suggests that depressive moods are socially 'contagious' because people pick up feelings of sadness, worthlessness and self-doubt from close others, but no research has tested whether this occurs over a longer time-span, such as changes in romantic couples' psychological wellbeing over years."
Fortunately, the longitudinal New Zealand Attitudes and Values Study (NZAVS), run out of the University of Auckland since 2009, offered Hammond and Professor Chris Sibley a way to see if these ideas indeed stacked up.
Over four years, 342 people in 171 couples, already taking part in the NZAVS completed questionnaires that asked them about their traits, beliefs and values. The questions traversed a range of aspects, such as life and body satisfaction, self-esteem, how liberal or conservative people were, and their attitudes toward climate change.
To tease out changes in the data accurately, the study team compared the growth-curve trajectories of both partners in a couple.
"Essentially, the test is: If we know that 'Partner A' increased across time, does that information make us more accurate at knowing how 'Partner B' changed across time," Hammond said.
Ultimately, they discovered that, while couples were generally similar when it came to their values and beliefs, there was evidence they were still different in personality traits, self-esteem, and body-satisfaction.
They also found that people's wellbeing and values could change – such as their acceptance of climate change – without that same shift necessarily being seen in their partner.
"People seemed to change in their own way, and didn't become any more similar to their partner across time - although equally, they didn't become the opposite of their partner either," Hammond said.
"In other words, if we know that 'Partner A' increased across time, that information does not help us know whether 'Partner B' increased, decreased, or stayed the same."
Hammond said these new insights proved a surprise, given that existing theories pointed to a clear trend to the contrary.
"We thought [people becoming more alike] would occur for people in long-term relationships and living together, since they share so much of their lives," he said.
"However, it may just be that people are romantically attracted to others who are already psychologically similar to themselves in the first place."
Whatever the case, he said more research was warranted.
"Because we were looking at people who were in long-term, established relationships, we don't know what happens in the early stages of relationships," he said.
"Maybe people change a lot in the first year of dating - becoming much more similar as they share new experiences or they debate the value differences that are personally important."