Long-term relationships are awesome, but it's not uncommon for the passion to burn out over time.

But researchers say they've found an evidence-based strategy that couples can use to keep the spark alive - and even relight it after it's gone out.

And it actually has nothing to do with sex - an international team of psychology researchers has shown that something they term "responsiveness" is linked to an increase in sexual desire among couples that are living together. 

Responsiveness is more than simply being nice - it's recognising your partners' unique needs and then meeting them, whether that's their desire to come home and have a glass of wine waiting for them, or to go out hunting Pokémon as a couple. 

The latest study hoped to test out something that psychologists call the "intimacy-desire paradox" - which is a conundrum that's puzzled scientists for decades.

Why does the thing so many of us crave (intimacy) seem to be associated with a decrease in sexual desire? And does that always have to be the case? Can we ever have great sex with a long-term partner?

The new research suggests yes (thankfully). But it's not necessarily intimacy, or familiarity with someone, that we crave, it's actually responsiveness.

In other words, we don't just want someone to be comfortable with, we want someone to see us for who we really are and respond to that. 

To figure this out, the researchers conducted three experiments on 100 long-term couples. First, they got participants to chat with what they thought was their partner online, but was actually either a responsive or unresponsive research assistant.

Secondly, the couple interacted in front of the researchers, and their responsiveness towards each other was measured objectively.

And finally, the couples were asked to keep a journal for six weeks to record their own level of sexual desire, as well as how responsive they felt their partner was being.

They also rated how special they felt and how valuable they thought their partner was as a mate each day.

The results showed a clear link between increased responsive and more sexual desire, especially for women, who were likely to feel more passion the more they felt their partner was responding to them. 

"When a mate is truly responsive, the relationship feels special and unique and he or she is perceived as valued and desirable," said one of the researchers, Gurit Birnbaum from the Interdisciplinary Centre Herzliya in Israel.

That's all well and good, but there are obviously a few limitations to mention here. For starters, the sexual desire was self-rated, and the researchers didn't actually measure how often the couples had sex.

A lot of couples still desire each other, but unfortunately life, kids, work, and routine gets in the way of actually having sex.

It's also a relatively small sample size by which to measure all the other couples out there. 

But it does give some pretty handy advice for anyone currently feeling like their partner isn't that passionate anymore, or who wants to make sure they keep the spark alive. Forget being nice for the sake of being nice, instead, be real.

"Being nice and things like that are not necessarily based on who the partner is and what the partner really wants," said Birnbaum.

It also gives long-term couples hope that they're not doomed just because they're past the first few years when everything is new and sexy.

"Sexual desire thrives on increasing intimacy and being responsive is one of the best ways to instil this elusive sensation over time; better than any pyrotechnic sex," Birnbaum added.

The research has been published in the Journal of Personality and Social Psychology.