It is acknowledged that some of the volunteers across different studies may have taken part in a study because they were more confident with their penis size than the general male population.
The study I linked seems to include both self stimulated erections and erections due to injection. They also limit themselves to clinical measurements. They mention self measured results but point out that they are unreliable, as you said. They do point out however that there might be a difference between self stimulation and an erection with a partner.
But all in all, there isn’t a barrier because of the ethics involved in touching a penis and masturbation.
A quick trip on Google scholar turns up a lot of studies on the size of male erections.
https://static1.squarespace.com/static/553598c1e4b0a7f854584291/t/55ee4a5ee4b025d99f73150e/1441679966732/Penis+Size+Study+-+Veale+et+al+2015+BJUI.pdf
Ha, poisoned data tho
Sure, they exist, but they have the flaws outlined above.
The study I linked seems to include both self stimulated erections and erections due to injection. They also limit themselves to clinical measurements. They mention self measured results but point out that they are unreliable, as you said. They do point out however that there might be a difference between self stimulation and an erection with a partner.
But all in all, there isn’t a barrier because of the ethics involved in touching a penis and masturbation.
Of course it was biased, those numbers are huge on there, it was men confident in their size skewing the data, at least that’s what I will tell myself