If you're a woman, there's a good chance you've received an unsolicited dick pic.
A 2017 survey found 53 percent of US women aged 18 to 34 have had a man send them a photo of their penis - and 78 percent of them have never asked for one.
Why are men so prone to sending explicit pictures to women out of the blue? CNN conducted an extensive investigation into the modern-day phenomenon, producing a list of possible explanations for the prevalence of unwanted dick pics.
One theory is that men feel emotionally attached to their penises.
"I think men, generally more than women, feel a connection to their genitalia and want the person they are interested in sexually to share that interest," New York psychotherapist Jeannette Stern told CNN.
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She also thinks many of the men who send dick pics tend to lack impulse control - especially if they've been drinking.
"Someone who is more impulsive is more likely to actually send the photo, especially under the influence, whereas a less impulsive person might want to but then thinks better of it, unless perhaps under the influence."
Alexandra Katehakis, founder and clinical director of the LA Centre for Healthy Sex, says some men are just "bad" at gauging whether or not women are interested in them.
However, she also says some men know full well that their dick pic is unwanted, and send them anyway as an act of "sexualised hostility".
"For the man, it's very much about power and control," she told CNN. "Men taking out their rage against women in an erotic form."
Sex therapist Joe Kort agrees with her hypothesis, saying some men get a thrill out of shocking women this way.
In 2019 when just about everyone has a smart phone, some men take advantage of modern technology to get their kicks.
It's easy to send an image to a stranger through the iPhone's AirDrop feature, and this is perfect for those who want to send a dick pic to the woman at the next café table or sitting opposite them on the train.
It's a phenomenon known as 'cyberflashing' or 'dick-dropping', and some US lawmakers have suggested making it a crime.
"It's not that different from old school exhibitionism," Ms Katehakis told CNN. "We think of the creepy guy in the trenchcoat. This is the modern-day version of this."
Sex therapist Russell Stambaugh theorises that the dick pic is an act of dominance - something men are more prone to seek than women thanks to traditional gender roles.
"Because it is often unsolicited dominance seeking, guys' genital pics are far more often regarded as aggressive," he told CNN.
He also believes men may be hard-wired to do things like send erotic photos because of something called "sexual signalling", in which humans try to simulate the conditions that result in reproductive behaviour.
Ms Katehakis also thinks there may be some evolutionary logic behind dick pics - but says that's no excuse.
"It probably has some biological underpinnings, but we've developed these big brains to negate all that."