If you've ever filled out a Census form, a college application or a patient questionnaire at the doctor's office, you've probably been asked to identify your race and ethnicity.

Governments, workplaces and educational institutions often collect data on these categories to determine things like which programs require funding, what disparities exist between different groups and when civil rights violations are occurring. But you might have also felt that checking a box on a form requires you to define yourself in ways that don't necessarily align with your own identity.

If it seems like the distinctions between race and ethnicity are confusing, unsatisfying or unclear, you're onto something. These categories are messy and lack concrete definitions. Their meanings have evolved over time and can shift depending on the context.

"It's not like there is some truthful race and truthful ethnicity out there, and that we bestow it on the population," said Tomás Jiménez, a sociology professor at Stanford University who studies race and ethnicity. "It comes from an observation of how people use these ways of categorizing themselves and each other."

Put another way, race and ethnicity are social and political constructs. Still, they carry enormous consequences in the US, Jiménez and other scholars say. Here's how to make sense of them.

Race and ethnicity, defined (sort of)

In US parlance, race refers to a group of people who share physical traits -- such as skin color, hair texture or eye shape -- based on some common ancestry. That common ancestry is broadly related to geography, said Grace Kao, a professor of sociology at Yale University. (For example, White people can generally trace their roots back to Europe, while Black people can generally trace their roots to Africa.)

The US Office of Management and Budget, which determines the racial categories used by the Census Bureau and other federal agencies, currently outlines five racial groups: American Indian or Alaska Native, Asian, Black or African American, Native Hawaiian or Other Pacific Islander, and White.

Ethnicity, meanwhile, refers to a group of people who share a common history and culture. It sometimes (but not always) correlates to national origin -- for example, a person might be categorized as racially Asian and ethnically Chinese. But this understanding of ethnicity would not apply in other parts of the world. In China, for example, a person's ethnicity would be described using more specific terms. There are 56 officially recognized ethnic groups in the country, including the Han people, the Mongols and the Uyghurs.

Census forms and other questionnaires rely on self-identification to determine a person's race and ethnicity. But people make assumptions and assessments about others' racial and ethnic identities all the time, and individuals don't have control over how they're perceived. Nancy López, a professor of sociology at the University of New Mexico, refers to this phenomenon as "street race" -- the race that people see you as when you're out in public.

"When you show up to look for an apartment, people are not asking you 'What's your ancestry?'" López said. "They just take a look at you and decide, 'We want people who look like you living next to us or we don't.'"

One way to understand street race is through the way Blackness is characterized in the US. Someone can have a Black parent and a White parent, but if they have a certain skin tone and hair texture, they will likely be perceived as solely Black (a legacy of the one-drop rule that classified anyone with known African ancestry as Black).

Kao pointed to her own identity as an Asian American woman as another example. People might assume upon looking at her that she doesn't speak English or that she's an immigrant. But she said her husband, who is White and from Canada, doesn't face those kinds of assumptions.

"That speaks to how important race is," she said. "It's not something we can just pretend doesn't exist, because it affects everything in terms of our daily lives."

Examples of how race and ethnicity overlap

Despite their importance in our society, the categories of race and ethnicity are far from fixed.

Merriam-Webster's dictionary entry shows that race has previously been understood as "a group of people sharing a common cultural, geographical, linguistic, or religious origin or background," "the descendants of a common ancestor" and "a group of people sharing some habit or characteristic (such as profession or belief)." During the late 19th and early 20th centuries, for example, groups such as the Irish, Italians and Jews were referred to in the US as separate races. Today, members of those groups would largely be classified as White in the US.

The US Census is another useful case study in the malleability of race.

Beginning in 1870, the government added Chinese under a category labeled "color" to describe all East Asians. The "color" category was later renamed as "race," and Japanese was added in 1890. Later iterations of the form used the term Hindu to describe South Asians (despite the fact that most South Asian migrants at the time were Sikh and Muslim), according to the Pew Research Center.

Mexican was included as a racial category on the 1930 census. But Mexican American groups at the time didn't want Mexicans to be counted as a separate race, fearing they might be targeted by the government. It would be 40 years before the government tried to count the Latino population again, this time asking about origin separately from race. Though the wording of the question has since evolved, the census continues to categorize Hispanic and Latino as an ethnic identity rather than a racial one.

The five racial categories that are listed on the census today -- American Indian or Alaska Native, Asian, Black or African American, Native Hawaiian or Other Pacific Islander, and White -- have been in place since 1997. But there has long been debate about whether those are sufficient, and the Biden administration is currently proposing changes to the 2030 census that would affect how certain populations are counted.

The proposal would combine the race and ethnicity questions into one. That means instead of being asked about Hispanic or Latino origin separately from race, respondents would see a box for "Hispanic or Latino" alongside such categories as "Black," "White" and "American Indian or Alaska Native." Respondents would be able to select multiple categories from the list. Several Latino civil rights organizations support the change -- many Latinos currently check the "some other race" box, and there have been concerns about whether the population is being adequately counted.

Others, including Afro-Latino scholars such as López, argue that combining race and ethnicity into one category on the census would lump together a highly diverse population and make it more difficult to understand racial inequities in housing, employment and other arenas. There are White, Black and Indigenous Latinos, López said. She instead proposes that the Census add a category such as "brown" to capture the specific experiences that Latinos of mixed ancestry experience.

Under the proposed changes to the 2030 Census, "Middle Eastern or North African" would also be added as a category -- the government currently classifies those of Middle Eastern or North African (MENA) descent as White, although many Arab Americans have said that does not reflect their reality.

Kao said such debates over how groups are categorized illustrate just how complicated these categories are.

"Everything is messy," she added. "If we were talking five years from now or 10 years from now, it could be totally different."

Why race and ethnicity are important

Jiménez thinks about the categories of race and ethnicity as "claims that we make about who's in and who's out."

In the case of race, Jiménez says, society has ascribed meaning to certain physical features and created a hierarchy around them that informs who is treated with respect and dignity and who has access to wealth, education and other resources. But race isn't an innate biological classification -- anthropologists and geneticists who have studied these questions have found that there is no set of physical or behavioral traits that corresponds to all of the people in a given race. Our understanding of race is instead a product of colonization, the transatlantic slave trade and migration patterns, scholars say.

"Our modern conception of race is one that is European, and it immediately puts Europeans or Whites at the top of the hierarchy," Kao said. "You can argue that if you have racial groups, you don't need to have a hierarchy. We can say the world is divided into these populations and not assume that one is superior to the other. But that's not the way our racial categories have formed."

Ethnicity can be similarly squishy, Jiménez said. There are certain markers that we typically use to determine whether someone is part of an ethnic group, like where their family is from, what foods they eat or what language they speak. Still, a person with Puerto Rican parents who doesn't speak Spanish might be seen by others in the community as "not Latino enough" just as a person with one Iranian parent might be seen as "not Persian enough."

To complicate matters, ethnicity is sometimes conflated with political states that may not have existed in their current form a few hundred years ago. For example, Jiménez points out, Italian is widely considered an ethnicity in the US today. But some earlier Italian immigrants might not have described themselves as Italian given regional differences within the population, he added.

"The fact that the boundaries move around... (and) the fact that those change over time all speak to the fact that we're making this up collectively," Jiménez said.

Given that these categories are social constructions, it might be tempting to suggest that we do away with them all together. On an individual level, these classifications can feel limiting, Kao said -- no one wants to feel constrained by stereotypes or perceptions that others have of them because of their racial or ethnic identity. But at the same time, race -- and ethnicity to a degree -- has very real implications in our society, and understanding it is imperative to discern where disparities are occurring and how they might be addressed.

That, Jiménez said, is the paradox of these socially constructed categories.

"It is something that frustrates us," he said. "It's also in some ways a requirement if we are to get to a place where these categories matter less and less -- in ways that affect us negatively."