Look up any article about President Obama that focuses on his role as the first black president.

Go ahead, do it now.

Scroll down to the comments.

I promise you, you'll find earnest inquiries asking why the president is considered black or biracial when his mother is white. You'll find people who are sincerely saddened by the idea that he would "reject" her contribution to his heritage. You'll find people who are legitimately confused about why half black plus half white sometimes equals black and sometimes equals biracial, but rarely if ever seems to equal white.

Here's an example of someone wondering about this on Quora:

Here's a blog post on the same topic, taking more of an advocacy approach aimed at changing his public perception:

This is why multiracial people don't normally identify as white

A new study by Pew Research Center takes a comprehensive look at the experiences of multiracial Americans. Using a different approach than the census by taking into account people's parents' and grandparents' racial backgrounds in addition to their self-reported race, it concluded that multiracial adults currently make up 6.9 percent of the adult American population.

One of its many findings has to do with multiracial identity, and that age-old question of why mixed-race Americans like Obama and so many others don't seem to give their white parents' ethnicity the same weight as their other heritage when it comes to self-description.

The study revealed that people who identify as multiracial say they experience discrimination based on the part of their heritage that is not white. Here's how Pew explained it in the write-up (emphasis added):

For multiracial adults with a black background, experiences with discrimination closely mirror those of single-race blacks. Among adults who are black and no other race, 57% say they have received poor service in restaurants or other businesses, identical to the share of biracial black and white adults who say this has happened to them; and 42% of single-race blacks say they have been unfairly stopped by the police, as do 41% of biracial black and white adults. Mixed-race adults with an Asian background are about as likely to report being discriminated against as are single-race Asians, while multiracial adults with a white background are more likely than single-race whites to say they have experienced racial discrimination.

This echoes the way Obama has explained why he calls himself black. "I'm not sure I decided it," he once said in an interview with 60 Minutes. "I think, you know, if you look African-American in this society, you're treated as an African-American."

He later told PBS, "If I'm outside your building trying to catch a cab, they're not saying, ‘Oh, there's a mixed-race guy.'"

It's not just about discrimination

But it would be oversimplifying things to say that the many people who have one white parent and one parent of a different race but choose not to call themselves white are reacting entirely to day-to-day experiences with racism, like being unable to catch a cab.

According to Pew, other factors that help shape the way people label themselves, and even inspire some Americans to check just one box despite the fact that their parents each checked a different one, are the following: how they look or think they look, who raised them, and which race they identify with (which is no doubt shaped in part by their looks, their upbringing, and the discrimination mentioned above).

When asked why they don't identify as multiracial, about half (47%) say it is because they look like one race. An identical share say they were raised as one race, while about four-in-ten (39%) say they closely identify with a single race. And about a third (34%) say they never knew the family member or ancestor who was a different race. (Individuals were allowed to select multiple reasons.)

Whether it's good or bad that people feel obligated to categorize themselves at all is the topic of an ongoing debate. But what's clear — and what this new study further illuminates — is that while racial identity is informed by a lot of things, mathematical equations that slice up heritage into equal proportions divorced from their social context aren't anywhere near the top of the list.