Scarlett Johansson has given boyfriend Josh Hartnett cause for concern, by branding monogamy unnatural and questioning romances between actors.

The screen stars have been dating since April (05) after meeting on the set of The Black Dahlia, but Johansson's comments have sparked questions.

She says, "I don't think human beings are monogamous by nature. It's difficult - you have to put a lot of effort into a relationship.

"I think it's hard for actors to date each other because they are so damn moody. You are away from people constantly and having a relationship that is strictly by phone, it is miserable.

"Or if you say to him/her, 'Hey, (even though) I am doing a very sexy scene with this very sexy girl/boy, I love you and I'm going to be thinking of you when I am rolling around in bed with this person!'"