How long does a sex session 'normally' last? And why all the thrusting and bumping?

I know there's a lot more to sex than putting the penis into the vagina and ejaculating, but the rest is not always easy to define - kissing? rubbing? grinding? Measuring an average time to ejaculation is not a straightforward matter.

Sex isn't something people normally do while monitoring the bedside clock, and unassisted time keeping may be difficult during a transportative session of love-making.

The best study we have estimating the average time to ejaculation in the general population involved 500 couples from around the world timing themselves having sex over a four-week period - using a stopwatch. That is as awkward as it sounds: participants pressed "start" at penile penetration and "stop" at ejaculation.

You may note this could affect the mood somewhat, and might perhaps not exactly reflect the natural flow of things.

The most striking result is that there was a huge amount of variation. The average time for each couple ranged from 33 seconds to 44 minutes.

So it's clear there's no one "normal" amount of time to have sex. The average across all couples, though, was 5.4 minutes.

There were some interesting secondary results, too. Condom use didn't seem to affect the time, and neither did men's being circumcised or not, which challenges some conventional wisdom regarding penile sensitivity and its relationship to staying power in the sack.

Another surprising finding was that the older the couple, the shorter the sex.

Why do we have sex for so long?

You might have once asked yourself after a disappointingly quick intercourse, "how long does sex 'normally' last?"

As an evolutionary researcher, all this talk of how long sex lasts makes me wonder: Why does it last any time at all?

All sex really needs to achieve, it seems, is to put sperm into the vagina. Why all the thrusting and bumping? Instead of sliding the penis in and out, why not just put it in once and ejaculate?

Despite the 'in-out' being fun, does it serve a purpose biologically?

In 2003, researchers showed that the ridge around the head of the penis scoops out pre-existing syrup from the vagina.

What this suggests is that men's repeated thrusting might function to displace other men's semen before ejaculating, ensuring their own swimmers have a better chance of reaching the egg first. This could explain why it becomes painful for a man to continue thrusting after ejaculating, since that would risk scooping out his own semen as well.

Zietsch is Research Fellow, the University of Queensland. Source: theconversation.com