Light skinned folks are zone 6 (1 stop over middle gray), and most dark skinned folks are zone 5 (middle gray). Very, very dark (I mean living right on the equator dark) skinned folks approach zone 4 (one stop under middle gray). If you are metering off a black guy's face, use the recommended reading; if you are metering off a white guy's face overexpose one stop.
Compare your background reading to middle gray. Pure white is 4 or 5 stops brighter than middle gray.
At least that's how it should work. Now I'm going to talk about something the camera industry doesn't want you to know. Many in-camera meters don't measure middle gray, they measure zone 6 (one stop brighter than middle gray) I've heard this called the "K factor". I don't know why it's called that. It occurs more often in entry level SLRs and point-n-shoots than fancier, pro-model cams.
The reason is that the industry demographics indicate that most cameras are used to shoot face/head portraits of people the camera owner knows, and whites and light skinned asians buy more cameras. So they set the meter to measure one stop over middle gray, because if you meter off a white guy's face for middle grey, you'll be underexposing one stop.
The only way to know for sure if your camera is set this way, because your camera manufacturer sure isn't going to admit this, is to compare your in-camera readings to the reading made by a hand held meter.
This is the kind of thing you got to hope has been done away with, but it wouldn't surprise me if it's still going on. I use so many different cameras that I went to a hand held meter many years ago, and don't pay much attention to in-camera meters. So I don't know what's up these days. When I compare most of my 60s, 70s, and 80s in-camera meters to my hand held model, they all overexpose by about a stop.
Now I suppose I should make the disclaimer that I'm a cynic when it comes to the way folks treat each other. Some people might explain this whole thing as the camera manufacturers just wanting to make sure you get details in your shadows, but I've heard this from three completely different sources, and none of them were internet. I believe it, but like I said, I'm a cynic.