When a friend says she thinks the average pen*s size is 'like seven inches'

Do you know if there are big differences across ethnic groups? Or is it all empty stereotypes?

Yes, big differences .... From what I hear

All dick sizes are roughly the same, regardless of ethnicity.

Thank you, anonymous stranger.

I did a bit of looking around this afternoon and it doesn't look like any reputable studies have really been done. At least at a glance, the internet is full of contradictory opinions but basically zero data.

So, all stereotypes start out of a grain of truth. There's some trait or thing that a group of people have or do that starts the stereotype. Sometimes it gets exaggerated a lot more than other stereotypes, but it still becomes one for a reason.
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To lay it out simply for you, yes the average asian dick is smaller than the average black dick. Doesn't mean you won't see some asian dude with a 12inch or black dude with a 4 inch, but on average.

I see a ton of cock at my job (among other things), and I can honestly tell you that there's very little difference with regard to race.

...so... what exactly is your job that you [get to] look at cocks all day?

Orderly. I worked forever as a float, but now I just work the er.

And you get a lot of genital injuries? What's the most common cause of damaged wang, in your experience?

You misunderstand. Very rarely is the wang actually injured. Sometimes it's something lame like a guy purposely leaving himself exposed thinking a female nurse is going to wander in. Sometimes it's just someone we've had to cut out of their clothes while we attempt to resuscitate them. Then there's the good 'ol clean-up work we have to perform dozens of times per day since we constantly get seniors from nursing homes where they aren't cared for particularly well, or are simply incontinent.
Oh, and we've been known to get a testicular torsion on occasion.