By Michelle Roberts
BBC News health reporter in Copenhagen
A simple brain scan can spot whether a woman is faking an orgasm or not, a major fertility conference has heard.
Tell tale brain activity in a fake orgasm
Researchers found that when a woman is faking, a part of the brain under conscious control lights up, while real orgasms occur subconsciously.
The University of Groningen scans also showed important differences between the male and female orgasm.
In women, turning off fear and anxiety is key, while men need to know they will be physically stimulated.
Professor Gert Holstege and colleagues asked 13 heterosexual couples aged 19-49 to take part in an experiment.
One half of the couple was asked to lie down, with their head inside a scanner, while their partner stimulated them manually to achieve orgasm.
To aid the mood, the room lighting was dimmed and all noise distractions shut out.
The couples then switched positions and the experiment was repeated.
One thing that they found was putting the couples off the task was literally cold feet.
Genuine orgasm: less brain actvity
When they gave the couples socks to wear, about 80% of the couples were able to achieve orgasm compared with 50% previously in this staged environment.
The women were also asked to fake an orgasm so that these scan results could be compared with those taken during genuine orgasms. There were obvious differences.
Professor Holstege said: "Women can imitate orgasm quite well."
But with genuine orgasm, he said: "What we see is an extreme deactivation of large portions of the brain and especially the emotional parts involved with fear.
"If you look at the women who faked orgasm intentionally you see the motor cortex - the conscious part of the brain - is activated.
"This means the movements that we make in [real] orgasm are not conscious.
"And if you are fearful, it is very hard to have sex. It's very hard to let go."
He said this was useful for men to know.
"When you want to make love to a woman, you must give her the feeling of being protected."
In comparison, the scan results suggested that for men knowing that they would be physically stimulated was important because areas of the brain involved in interpreting touch were highly activated during the experiment.