If you've ever been on the receiving end of unsolicited photos of male genitalia, you've probably questioned what in the name of God compelled the person on the other end of the phone to do it.

Well, according to a sex and relationship expert, the inclination stems from one of two things; the desire to procreate and the desire to shock.

With a recent study revealing that 47 per cent of men have sent an unrequested pic of their bits, Dr Justin Lehmiller weighed in on the practice and provided the public with two theories for this behaviour.

The first theory focuses on men's desire to procreate, and highlights some men's inability to differentiate between friendliness and flirting.

The theory suggests that when a man overestimates a woman's interest in him, his subconscious considers procreation and this ultimately helps him to rationalise exposing himself.

"Although this strategy means men are necessarily going to fail a high percentage of the time, it requires a relatively low investment on their part while ensuring that they don’t miss out on anyone who might be interested," writes Dr Lehmiller.

Dr Lehmiller stresses that while this behaviour can be explained on some levels, it doesn't justify the practice.

"Let me be perfectly clear about one thing: even if we think of this behaviour as being adaptive in an evolutionary sense, this isn’t to say that it’s OK or excusable for men to send women photos that they don’t want to see."

The second theory is (perhaps unsurprisingly) based on the concept of exhibitionism.

According to Dr Lehmiller, some men hope their snaps will lead to sex while others simply get a kick out of eliciting a shocked response from recipients.

"Although most people think of exhibitionism in terms of guys who flash strangers on the subway or in a park, a similar kind of behaviour can also occur online or over the phone," he writes.

"This behaviour is linked to having poor social and interpersonal skill – as such, some psychologists believe that exhibitionistic tendencies develop following the inability to establish relationships in a more conventional way," he adds.

And now we know.