Sorry men - size really does matter. And bigger is immeasurably better.

In a new and definitive scientific study on penis size sure to have global repercussions, a team of Australian National University researchers have put male genitalia under the microscope.

In a nutshell, the research shows that penis size directly correlates with a man's attractiveness to women. For some that may be bad enough news, but it gets worse - even the longest penis inspected by the women in the study may not be big enough.

PERFECT?: Research has already confirmed that women generally prefer taller men and men with broad shoulders and narrow hips.

Professor Michael Jennions, an ANU behaviour and reproductive biologist, and his PhD student Dr Brian Mautz, will on Tuesday publish in the American journal Proceedings of the National Academy of Sciences the most comprehensive scientific investigation into the importance of penis size.

The long and the short of it is women display a clear preference for penises in excess of 13 centimetres in their flaccid state. This is beyond the measurements of 95 per cent of men.

Professor Jennions, whose previous studies have focused on the sex lives of fish, crickets and fiddler crabs, decided in 2012 to fill a void in scientific research on the importance of human penis size to attracting potential mates. "We seem to know every aspect of a woman's attractiveness to men - from breast size, to waist-to-hip-ratio to leg length,'' Professor Jennions said. "I can't help but wonder whether this is a result of there being so many male scientists.''

He and Dr Mautz recruited 105 Canberra women in their prime reproductive years - aged between 20 and 40 - and asked them to assess 53 fully life-sized, 3D video images of naked men.

The women had no idea what the study was about when they were asked to rank each man's sexual attractiveness.

The team used natural variations in body shape and height based on one of the largest comprehensive physical measurement sample ever produced - which just happens to be of 3300 young Italian men. They then created composite images ranging in body shape, height and penis measurements of between 5 centimetres and 13 centimetres, representing the shortest and longest penis lengths in the Italian sample.

"I guess there are going to be a lot of guys out there with rulers today. Not that they don't already know instantaneously,'' Professor Jennions said.

And how big was too big? One of the most surprising aspects of the study was it could not accommodate an end point at which women found any penis too large. "Our curve wasn't going down at 13 centimetres so we did not reach the most attractiveness in terms of size. The concept of a penis that was 'too big' was outside the range that we tested and we simply didn't find an absolute maximum size, even though it strikes me that it is going to start to look ridiculous and actually quite painful.''

Previous research has already confirmed that women generally prefer taller men and men with broad shoulders and narrow hips. V-shaped bodies, rather than pear-shaped.

But the ANU has taken that assessment further, finding that penis size is just as important as height when it comes to attracting women.

Fittingly, larger penises were much more attractive on taller men than shorter men.

"To put it bluntly, if you are short and pear-shaped a large penis is not going to increase your attractiveness,'' Dr Mautz said.

The women were also brutal in their judgments - sizing up a naked man before rejecting him as unattractive in three seconds flat.

And they were even quicker when the penis was small.

"Now that's quick,'' Dr Mautz said. "That's so quick that you can't really, in that short space of time, consciously go through the pros and cons of each character; that rating is an overall impression of attractiveness, not focused on any particular body part.''

This was an important aspect that distinguished the team's research from the one previous study of the effect of penis size on attractiveness in humans. The New Zealand study, begun in 2006, used five small drawings of male figures that differed only in penis length.

"It was very obvious to the female participants which character they were being asked to assess; in that situation, it's easy for people to self-deceive or lie and distort the results,'' Dr Mautz said.

"Not only did we vary several body parts at once, we also used life-size, 3D figures, which is much more realistic than simply seeing figures on a piece of paper.''

Professor Jennions said the study was not just an amusing topic of interest to the public, and endless fodder for women's magazines, but also a valid scientific question.

"Humans have slightly longer and notably thicker penises for their body size compared to other primates. It's been suggested that before humans wore clothes, females used penis size as one of the deciding factors when choosing a mate. Until now there has been very little data to support that. Our results hint that humans might have larger penises because females in the past were more likely to mate with well-endowed males,'' he said.

"We may debate whether a man is attractive when he has kind eyes or a good sense of humour, but now, at least, we know the truth.''

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