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How long? All you need to know about the perfect penis measurementsistock

Much research has been dedicated to finding out all things penis-size related. Here are some key studies into this area of science.

Average penis length is just over five inches

In March last year, a King's College London study published in the British Journal of Urology International found the average penis length – when erect – is just over five inches, or 13cm. The team was looking to find out what was "normal" in terms of length and circumference as it could help provide counselling for men concerned about it.

The scientists used 17 studies with measurements of over 15,500 men to determine average sizes when flaccid was 9.16cm in length and 9.31cm in circumference. Erect averages 13.12cm in length and 11.66cm in circumference. They added there was a small correlation with height, but not with shoe size.

Women want just above average for long-term partners

Women prefer larger-than-average penises for one-night stands, but when it comes to long-term partners they want just above average, UCLA and the University of New Mexico scientists found last year. Using 3D models to assess penis size, women selected from 33 models, indicating their size preferences for both categories of partners.

"Women preferred a larger penis size (especially a larger circumference) for one-time partners as compared to long-term partners," they wrote in the journal PLOS One. "While this preference for a larger phallus is above the average penis size, it is only very slightly above the average."

In terms of ideal size, the average preferred length for a long-term partner was 6.3 inches in length and 4.8 inches in circumference. This increased to 6.4 inches in length and five inches in circumference for one-time partners.

Penis size and body shape influences male attractiveness

In 2013, researchers from the Australian National University looked at the link between penis size and body shape and how it relates to male attractiveness. Publishing their findings in PNAS, they found women preferred men who were tall, had broad shoulders, narrow hips and a large penis.

They did this through computer-generated images showing a range of body shapes, height and penis size (between 5cm and 13cm). The most attractive penis size was found to be larger than 95% of men, and that this was just as important as height – a well-known factor in male attractiveness, study author Michael Jennions said. The team also found larger penises were seen to be more attractive on taller men: "To put it bluntly, if you are short and pear-shaped, a large penis is not going to increase your attractiveness," fellow author Brian Mautz said.

Finger length may be linked to penis size

A 2011 study by scientists from the Cachon University in Incheon, Korea, found that men whose index fingers are shorter than their ring fingers appear to have longer penises. Published in the Asian Journal of Andrology, the team looked at digit ratio and penis size of 144 men. Results showed that after accounting for age, weight, height, BMI, finger length and digit ratio, only height and digit ratio were associated with penis length.

The team believe this could be something to do with the influence of testosterone in the womb.

"Based on this evidence, we suggest that the digit ratio can predict adult penile size and that the effects of prenatal testosterone may in part explain the differences in adult penile length," they wrote.

Having a larger penis may mean your wife is more likely to cheat

Having a larger than average penis may mean your wife is more likely to cheat, a 2014 study published in the journal PLOS One found. An international team of researchers studied extra-marital affairs among people living along Lake Victoria in Kenya.

They found women were more likely to have affairs if their husbands had larger penises, and that size correlated with the risk of cheating. "Surprisingly, [having a] spouse [with a] longer fully erect penis was associated with increased likelihood of the women having extra-marital partnerships," they wrote. "From these results, every one inch longer penis increased the likelihood of women being involved in extra-marital partnership by almost one-and-half times.

"Similarly, our qualitative data also support this finding. Women associated large penises with pain and discomfort during sex which precludes the enjoyment and sexual satisfaction that women are supposed to feel."