Do you like Coke while your partner prefers Pepsi? Beware, the difference in brand choices can affect our happiness in relationships more than shared interests or personality traits, suggests a research.

"People think compatibility in relationships comes from having similar backgrounds, religion or education," said Gavan Fitzsimons, a professor at the Duke University in North Carolina.

"But we find those things don't explain how happy you are in life nearly as much as this notion of brand compatibility," Fitzsimons added.

The findings showed that partners who had low power in their relationships -- those who do not feel they can shape their partner's behaviour -- tend to find themselves stuck with their partner's preferred brands, leading for unhappiness.

"If you are lower in relationship power and have different brand preferences than your partner, you're probably going to find yourself stuck with your partner's favourite brands, over and over again. This could lead to a death-by-a-thousand-cuts feeling," explained Danielle Brick from the University of New Hampshire.

"Most couples won't break up over brand incompatibility, but it leads to the low power partner becoming less and less happy."

For the study, published in the Journal of Consumer Research, the team conducted experiments in several settings. However all produced the same result.

The researchers used brand preferences in soda, coffee, chocolate, beer and automobiles to study individuals and couples, some of whom were tracked over two years. These results were combined with findings on relationship power and happiness.

"It's an extremely robust effect, we found it over and over and over again," Fitzsimons added.