Cultural appropriation

Cultural appropriation occurs when one group adopts some practices or styles from another group's culture and uses them outside of their original context, sometimes in a demeaning manner. It's not inherently a bad thing, but it can become one if done in an inappropriate manner or without consent.

Overview
To illustrate the problem, consider all the awards that are given out, from military decorations to Nobel Prizes. It's a criminal offense in many jurisdictions to wear a uniform or medals without permission. These vary of course, but many awards and similar are considered "sacrosanct", and having a bunch of people running around with fake versions cheapens the impact of the real thing. Now consider the eagle feathers and war bonnets of Native Americans, which serve a similar purpose; a person wearing these as a cheap Halloween costume is doing the equivalent of wearing a bunch of Medals of Honor or Purple Hearts.

Of course, not all cases of a privileged group's borrowing constitute appropriation. More often than not, the originators are actually very happy to see outsiders respectfully sharing in their customs, which may allow both groups to better understand each other. Throughout history people have copied other cultures and have learned useful ways of doing things from other cultures. Humanity has benefited when people learn from different cultures. All cultures have features that are worth repeating, and any culture that is "superior" is one that has learned much from other surrounding cultures. Most cases where people take up ideas or ways of doing things from other cultures happen uneventfully with no one getting upset or angry. Defining the boundary between appropriation and appreciation can, however, sometimes be tricky.

A further issue is when the dominant culture displaces the original one in popular culture, because some part of it was deemed "cool" it can get whitewashed. Caribbean pirates are cool, right? We shouldn't feel too bad about hurting the feelings of a bunch of murderous criminals on a boat, but were you aware that historically, not only were there were Black and Hispanic pirates, but combined they made up literally half of all pirates? After all, the Spanish and Portuguese did dominate the Americas for a century before the rest of Europe showed up. Yet in Western media, the overwhelming majority if not the entirety of pirates are depicted as White.

Some examples

 * Native American war bonnets worn as a "funny" item. War bonnets are reserved for the most respected members of the tribe and worn only on ceremonial occasions.  It's somewhat like wearing a pope hat in the Vatican.
 * Similarly, the iconic sun symbol found on the (and practically everything else that's remotely New Mexico-related) is held sacred by the  from which it originates. They generally do not mind its use, but appreciate it if people request permission first and have likened its misuse/appropriation as similar to "[seeing] an image of Our Lady of Guadalupe in an advertisement on the side of a portable toilet."
 * Depending on the context, white people with natty dreadlocks. While largely associated with Rastafarinism, dreadlocks have a long history in African culture. They have been worn in European cultures, for instance in Archaic Greece and possibly among Vikings, and Native Americans even developed them independently. Still, these examples are nowhere near as common as how often they were worn in African tradition.
 * Speaking of which, people pretending to be Rastafari to justify smoking weed. Genuine Rastafari have a prohibition on consuming alcohol and maintain strict vegetarian (sometimes including fish) i-tal diets, which most of the pretenders probably wouldn't be prepared to follow. Of course, the core of Rastafarianism itself was culturally appropriated from Judaism without permission, so it's a sort of karma.
 * Celebrities dressing up as someone exotic in music videos purely for the visual appeal, possibly referencing racist stereotypes in the process.
 * White rappers imitating the style of Black rappers wholesale, down to adopting a Southern African American accent. It's not an example if the White rapper and thus grew up with the same culture as Black rappers, but it is an example if   where none of the local White or Black populations have a notable connection with American hip-hop culture.
 * Most "ethnic" photo shoots.
 * Most tattoos in a language the owner can't read. Bonus points if the tattoo is a terrible Internet translation, or just nonsensical gibberish that only looks vaguely like the language. For that matter, general use of foreign languages for exoticism value rather than any attempt to actually convey information in that language. Or even worse, fonts/typefaces meant to look like stereotypes of what another language "looks like".
 * Similarly, trying to imitate Russian by writing in English with some Latin letters replaced by similarly-looking Cyrillic ones (without caring how they actually sound). Examples include "SРЕЕФОШЕТЕЯ" for "speedometer" from Don Bluth's Anastasia and "СНДИGЭ" for "change" from a dumb right-wing poster.
 * Similarly, African Americans often resort to Swahili words (Kwanzaa, uhuru, etc.) because Swahili is one of the more widespread and available African languages. Ironically, Swahili has been heavily influenced by many non-African languages, containing many Arabic, Hindi, Persian, and Portuguese loan words, originally written in an Arabic (Semitic) script, and currently written in the Latin alphabet, not to mention that Swahili is spoken on the other side of the continent from the areas most Africans in the New World had their roots (where said ancestors would have spoken Yoruba, Kikongo or other Western Bantu or Bantoid languages, Ful, etc.).
 * More of a literal example, but the British museums looting preserving various artifacts from throughout the Empire probably counts. While incredibly important discoveries such as the were used as building material before the French British found them, many of the artefacts were being preserved by the locals before the British stole them. In many cases, they were burial artefacts and we can assume that the original owners would not want them disturbed; imagine if your grandfather's grave was dug up 500 years from now and his bones, clothes and anything he wished to be buried with ended up on display in another country.
 * Speaking of Egypt, mummies being used as Halloween costumes is technically an example, but since the ancient Egyptian religion is long gone — outside of neopaganism, another appropriator — no one really cares.
 * Performing haka if you're not Maori (such as on the University of Arizona football team), unless you're a member of the New Zealand All Blacks performing it in the authorized and correct manner.
 * Controversially, yoga. Although considered a physical exercise by many, it has its roots in Hindu spiritualism and philosophy. Some Hindus see yoga as practiced in the West as appropriated into another form of trendy fitness, dressed in New Age trappings. It is worth mentioning, however, that the current Indian government puts great effort into promoting yoga worldwide. So the question is — does it count as cultural appropriation, if it is driven by the original cultural phenomenon possessors?
 * The appropriation of the swastika as the symbol of the Nazi party, along with other pre-Nazi supremacist and nationalist groups, from the cultures of Troy and the Indian subcontinent.
 * Use of Native American terms and imagery amongst professional sports teams has been a long-running controversy. One such example was the Washington Redskins, whose name and logo were contested by some Native Americans and activists; they eventually dropped the name in 2020 after a lot of criticism.
 * Use of Maasai imagery by various companies, including Land Rover and Louis Vuitton to promote their products.
 * Cowboy culture was in large part Hispanic, words like "buckaroo" come from Spanish words like but you wouldn't get that impression watching some movies. Generally speaking, being a cowboy wasn't the glamorous job of gunslinging, but rather the drudgery that is ranching and farming.
 * During the Middle Ages and Renaissance, many Christian grimoires incorporated Kabbalah and other forms of Jewish mysticism, despite the virulent Anti-Semitism that Europe harbored at the time.
 * The New Age movement is notorious for taking concepts from Eastern and Native American spirituality, stripping them of all nuance, and then insisting that their bastardizations are more accurate than modern science, thereby reinforcing the stereotype of non-Western cultures as ignorant, superstitious primitives.

Tiki and the Pacific Islands
Tiki was a major commercial enterprise in the United States from the 1930s through into the 1970s, but with a brief revival starting in 2008. The Tiki bar style appropriates widely from disparate Pacific Islands, with gaudified worst-of-Chinese-American-style cuisine, often resulting in an offensive mess of nonsense.

The Tiki bar and Tiki style were invented by two White Californians in 1933-1937: Donn Beach ("Don the Beachcomber") and Victor Bergeron ("Trader Vic"), who appropriated both the Maori legendary first ancestor named Tiki, and Chinese cuisine. Tiki sat within a much wider range of Pacific Island cultural appropriation. The tiki torch has been made continuously since the 1950s, and was further appropriated by the alt-right during the Unite the Right riot. The sculpture "Der Neue Mensch" by Otto Freundlich was inspired by the Easter Island moai sculptures and was featured on the cover the catalogue of The Nazi Degenerate Art Exhibition.

Transethnicity
Related to cultural appropriation is the concept of transethnicity or trans-race, something infamously associated with Tumblr users (though many of these accounts could have been 4chan sockpuppets). Transethnicism is the self-identification of oneself as another racial or ethnic background (or to put it bluntly, "transgender, but with race"). More often than not, it takes the form of a Japanophile conflating their enjoyment of anime and Pocky with actually being Japanese, or a New Ager conflating their spiritualism, 1/256 blood quantum, and having a chic dreamcatcher keychain with actually being Native American. This, like other "dysphorias" that toe the Poe line (transableism, trans-fat, etc.), is generally strongly looked down upon.

In the real world, transethnicity, broadly, refers to people who were adopted and raised by parents who come from and live in an ethnic or cultural background different from their own (for example, a passing white child being raised in a black or First Nations family, a Japanese orphan being raised by Chinese adoptive parents in Beijing, or a dark-skinned urchin — thought to be Roma — ). It is a relatively common and complex issue of racial and ethnic identity in the modern globalized world, that is, unfortunately, misused.

Someone's ethnic/cultural identity might also change when you immigrate and assimilate into another place than your own cultural background (a different country or even just a different town but with an ethnic minority population). You could have a unique identity combining both cultures, which is how a group is born. A child born from immigrants is called a.

Stealing from dominant cultures
Another issue is where you're not stealing from an obviously oppressed minority. Scottish Highland culture (kilts, tartan, Highland dress, dancing, Gaelic language, etc) has suffered in the past due to oppression from southern Scotland and the British government. But now that Scots are not oppressed (sorry, Scottish National Party, they're not), is wearing a kilt cultural appropriation? Similar issues occur with people in North America claiming Irish ancestry (Irish people were subject to racism and British colonialism, although claims that Irish were actually slaves is itself kind of racist or appropriative). Generally there doesn't seem to be 100% consensus, but many people are happy to see fashions and traditions spread more widely as long as nobody is being a dick.

On the other hand…
Arguably, not all instances of appropriation ultimately result in increased marginalization. For example, white musicians imitating the style of rock'n'roll, blues, and jazz musicians in the 1950s actually broadened the audience for these styles and helped Black music come to prominence; likewise, Alfred Reed's unambiguous borrowing of Russian musical traditions as part of a (mildly successful) attempt to help out US-Soviet relations can hardly be considered harmful. A little bit later, white women began to break the gender divide in these forms of music and made them more accessible to an even greater number of people. Culture does not work like a commodity market and imitation may bring the originals into the limelight. This effect happens all the time: for example, film adaptations tend to sharply increase the sales of the original books.

Also, in terms of the creative arts, a blanket prohibition on cross-cultural appropriation can actually be counterproductive, especially in environments where the lion's share of the artists are dominated by a majority culture, leading to segregation within said media.

It's best to do your research and be sure a cultural practice is original to a group as well. Belly dancing is common (even stereotypical) as part of Arab culture, but does not originate with it (something at least one Arab-American woman objecting to white women belly dancing did not know). The origin of belly dancing is obscure and might be anywhere from pre-Islamic North Africa to Asia Minor, Persia, or India, but it long predates both Islam and the expansion of Arab culture from the Arabian peninsula (although this doesn't necessarily mean white people should dress up like Barbara Eden in I Dream of Jeannie).

That being said, one thing turning out okay is not a very good argument for doing something else that could end up in marginalization.

Everything is cultural appropriation
On yet another other hand, humans have been "borrowing" or outright stealing ideas since the dawn of time. Alphabetical writing was likely only invented once by the proto-Semites and subsequently refined (through the addition of vowels) by the Greeks and all non-syllabic and non-logographic alphabets are based on those, so writing or reading this article is only possible because someone somewhere "borrowed" from a culture not their own. Similarly, pants were first worn by Celtic and Germanic tribes that were seen as barbaric by the Romans who wore more cumbersome garments. The problem with cultural appropriation is not the act of taking inspiration from another culture in itself, but rather the attitudes and discrimination associated with it. Wearing dreadlocks is not a problem in and of itself, but adopting various Rastafari icons just to smoke marijuana is deeply insulting to actual Rastas. Listening to rap or R&B music is not a problem, but ignoring the contributions of Black artists or consciously refusing to listen to it if it's from a Black artist is. "Dressing up" as other ethnicities is a problem, especially when you turn a holy symbol into a cheesy getup.

How to avoid

 * If you buy a cultural item, you may want to ask whether there are ways of using the item that are considered disrespectful. For example, while war bonnets are considered sacred, dream catchers are not. However, some consider the non-traditional use of dream catchers in other forms, such as jewelry or keychains, to be appropriation.
 * If you've been invited to or want to try an activity from another culture (such as yoga, capoeira, t'ai chi chuan, djembe playing, etc.), barring few examples, it's perfectly okay to learn or participate. Ideally, try to find an instructor or class, and don't be a twit about it.