Penis-measurers of the world, breathe easy: you have been validated. A new study, carried out by sexuality researchers at Indiana University, has found that the average erect penile size is exactly 14.15 cm (that's 5.57 inches). That number is notable not because it differs from previous studies, but just the opposite: it's quite in line with prior research, "suggesting that men likely self-report data accurately—or at least reliably—to research teams." (A choice excerpt: "Although many men may wish their penis were larger and may expend significant effort toward penile enlargement, this does not necessarily mean that men overreport their penile size to researchers.")
Cleverly, Dr. Debby Herbenick incentivized men to be honest, as she explains on Psychology Today:
Unlike most previous studies of self-reported penis size, [subjects] had good reason to report accurate data to us because we were using their size data to match them to a condom that was sized to fit their erect penis. If they reported a bigger-than-reality size to us, they would get a baggier condom. If they reported a smaller-than-reality size to us, the condom would be too tight.
Well played. Except the condoms were more than just motivational ploys. "We didn't set out to be 'penis size researchers,'" Dr. Herbenick explained in an email. "It was part of a larger study on condom fit—and the Discussion section of the study says a bit more about other fruitful areas of research."
Fruitfully enough, Herbenick's study produced another unexpected find. Unlike previous studies, this one took into account how men achieved the erection they were measuring. As it turns out, "oral stimulation of a man's penis resulted in reports of greater lengths and fantasy alone was associated with significantly smaller penile dimensions reported." This could suggest oral sex causes larger erections. Or—because correlation causation etcetera etcetera—it could mean men with larger penises are just more likely to be receiving oral sex in the first place.
Clearly, more research remains to be done. Which might be good news for penis-measurers.