Scarlett Johansson is convinced her relationship with fiance Ryan Reynolds will last, because they are among the few humans who can be monogamous.

The actress claims remaining faithful to one partner is "hard work" and not natural.

She tells the latest issue of Cosmopolitan magazine, "I don't think it's a natural instinct for human beings.

"Monogamy can be hard work for some people. I don't think it applies to everybody, and I don't think a lot of people can do it."

But Johansson insists she is among those who can resist the temptation to cheat: "I believe in finding a soul mate. I've always been in monogamous relationships. I would never want to be in an open one. It'd be too awful."

Reynolds and Johansson announced their engagement in May. The actress previously dated Josh Hartnett and Jared Leto.