text: The book Sex by Numbers: What statistics can tell us about sexual behaviour was released this week and it's a must-read for anyone fascinated by who really does what, where, when and how in bed. The author, David Spielgalhalter, a Cambridge University professor, sets out to investigate various sexual statistical claims and separate myth from reality. It's prompted me to do the same. Scroll down for video . Scientists who reviewed 20 different studies of penis size, including 15,521 measurements of penises from around the world found some surprising results. Since no other part of the human body is more shrouded in myth and folklore than men's most precious part, it seemed the penis was the most deserving subject to study. Here's what the latest research has to say: . So, it is really six inches? A new paper in the British Journal of Urology International is based on the results of scientists who reviewed 20 different studies of penis size, including 15,521 measurements of penises from around the world. Most importantly, it was based only on studies where the penis had been measured by a trained health professional (surprise, surprise: men aren't terribly honest or accurate when they measure it themselves). The result: most men fall within just a couple of centimetres of each other, with most measuring around 5.2 inches in length when erect with a 4.6 inch circumference. If he wants to DIY, the correct way to measure a penis is to use a cloth measuring tape, not a straight-edged ruler. He should measure while it's erect and measure the side facing his stomach, pushing the tape down to press against the pubic bone (so he's not measuring fat!). Tracey investigate various sexual statistical claims and separates myth from reality . The average time between penetration and him ejaculating (for heterosexual couples) is nine minutes. This was the result of a European study of 1,100 couples from 44 countries. Each couple attempted intercourse at least twice a week for eight weeks and used a stop watch, clicking it on the minute he penetrated and clicking stop immediately after ejaculation. Previous studies settled on between two or three minutes duration; another rated the 'ideal time' of intercourse for both men and women at between three and 13 minutes. The more he exercises, the stronger his erection. A new study in the Journal of Sexual Medicine looked at how physical activity affected erectile functioning. Men who exercised the most (18 hours a week) had the highest sexual functioning scores. Watching porn doesn't affect his ability to perform. Two reputable online surveys found no link between the use of pornography and erection problems, despite popular perception that watching too much porn leads to ED (erectile dysfunction). What does make sense (though, again not proven) is that men who have difficulties turn to porn as a way of experiencing sex without embarrassment. Great sex does not depend on a big penis. A study including opinions of 25 highly respected sex therapists identified eight elements that make up the best sex you can possibly expect. Guess what? The penis didn't even get a look in. The myth that big hands/big feet/big nose means a big penis has no scientific proof or evidence supporting it whatsoever. But height did make a difference: the taller the man, the longer his penis is likely to be. Bald guys have a stronger sex drive. This may well have legs. The myth says bald men are more virile than men with hair because they have more testosterone in their body (which can cause hair loss). The testosterone/hair loss link is proven. Baldness is caused when hair follicles become exposed to too much dihydrotestosterone (DHT) which is a chemical produced by the male hormone testosterone. Testosterone is also the chemical responsible for his libido. The average number of erections per day for most men is seven. But five of these occur while he's asleep. The penis has a safety-valve that ensures a man can't ejaculate and urinate at the same time. The average quantity of semen ejaculated is 3ml. The average speed of ejaculation is 28 miles per hour. The amount of blood in an erect penis is ten times more than when it's flaccid. It shoots from 9ml flaccid to 90ml erect. One testicle hangs lower than the other to avoid them getting squashed when he walks! Eighty-five percent of men find their left testicle is lower and larger. Find more fascinating sex stats and info in all of Tracey's books on amazon.
target: A Cambridge University professor has written a book about sex statistics .
  Sex by Numbers investigates what the real average penis size is .
  Tracey Cox takes a look at the surprising findings .