Is it unlawful race discrimination for a company to ban dreadlocks in the workplace? In a decision that has become a topic of debate among law professors, a federal appeals court said no last month. The case is so important because the court defined race as biology, emphasising “immutable characteristics” as the subject of anti-discrimination law. But for more than 75 years, scholars have understood that race is as much or more a matter of culture than it is about biological reality.

The decision in EEOC v Catastrophic Management Solutions is therefore built on quicksand — and it is a mistake to embrace it, even if on some level the result might seem like common sense.

In 2010, Chastity Jones applied to work as a customer service representative for Catastrophic Management Solutions. The job would not have involved any in-person contact with customers: she would be sitting at a computer in a call centre.

Jones, who identifies as African-American, arrived at the interview wearing a business suit with her hair in what the court called “short dreadlocks”. After the interview, a CMS employee asked Jones whether her hair was in dreadlocks. When she said yes, the employee explained that CMS could not hire her “with the dreadlocks” because “they tend to get messy, although I’m not saying yours are, but you know what I’m talking about”.

CMS had a hairstyle policy in place, but it did not mention dreadlocks. It simply said that “hairstyle should reflect a business/professional image. No excessive hairstyles or unusual colours are acceptable.”

The Equal Employment Opportunity Commission thought Jones had been the subject of race discrimination, and sued CMS. But a federal district court rejected the suit, and the United States Court of Appeals for the 11th Circuit agreed.

The essence of the court’s opinion was that for the purposes of Title VII of the 1964 Civil Rights Act, which prohibits employment discrimination, “race” can be defined biologically as a set of “immutable characteristics”. And hairstyle isn’t immutable, because you can change it.

Looking mostly at dictionaries from the 1960s, when the Civil Rights Act was passed, the court said “it appears more likely than not that ‘race’, as a matter of language and usage, referred to common physical characteristics shared by a group of people and transmitted by their ancestors over time.”

And it said that even if “today ‘race’ is recognised as a ‘social construct’”, what matters is what Congress thought race was in 1964.

Superficially, this result may seem reasonable. After all, hairstyles similar to or identical with dreadlocks have existed in many cultures and in many times and places throughout human history.

They can be seen on ancient Greek statues and contemporary Indian sadhus. Just about anyone, it seems, can wear dreadlocks. That would seem to make dreadlocks into an artefact of culture, not a feature of biological race. Thus, it might seem, banning dreadlocks in the workplace has nothing to do with [biological] race, at least so long as CMS bans whites and blacks alike from wearing the hairstyle.

Stanford law professor Richard Thompson Ford, a brilliant and influential theorist of race, seemed to buy a version of this argument. He endorsed the decision, writing that “if dreadlocks aren’t part of racial identity — if they are more like a fashion choice — then banning them is no different than banning purple hair or handlebar moustaches, neither of which is unlawfully discriminatory.”

Ford has argued that courts should not choose between biological and cultural definitions of race. But in a case such as like this one, there is no escaping the choice. Either the ban on dreadlocks is race discrimination or it is not.

And the problem with relying on “immutable characteristics” is that the court’s sharp distinction between a biological definition of race and a cultural definition of race evaporates on closer analysis. Yes, dreadlocks are part of a cultural choice that for some people, although not all, is associated with race. But being black, or white or Latino or Asian, is also cultural fact; or if you prefer, a cultural artefact.

There is no objectively determinable content to what makes someone black. It certainly is not skin colour or melanin. There are no “immutable characteristics”, as is made clear in the context of sex by the transgender movement and in the context of race by the progression of Michael Jackson’s colour transformation.

What makes someone black is therefore a complicated combination of factors, which includes our cultural interpretation of embodied physical realities that include parentage and history. Defining race in terms of biology or immutable characteristics is therefore not just outdated — it’s plainly wrong.

To see why this is true, consider the punchline of the 11th Circuit’s dreadlocks decision: “Discrimination on the basis of black hair texture [an immutable characteristic] is prohibited by Title VII, while adverse action on the basis of black hairstyle [a mutable choice] is not.”

Several things are wrong with this sentence. Maybe the most obvious is that hair texture cannot be separated from hairstyle. Is my natural hair texture the texture it has when I’ve combed it? Washed it? Used conditioner? Added a product?

As Ford writes, under the court’s holding, “dress codes can ban hairstyles that result from artifice, such as dreadlocks, but not hair textures that are natural, such as Afros”. That makes no sense. An Afro is a hairstyle, just like dreadlocks.

But the deeper problem is the way the court was using the single adjective “black” to mean two completely different things in the same sentence. When it said “black hair texture” it meant a texture of hair that is statistically, or stereotypically, associated with people descended from Africans. When it said “black hairstyle”, it meant a hairstyle statistically or stereotypically associated with African-American culture.

In other words, the court is trying to use the word “black” to refer one time to a biological concept of race, and the other time to a cultural concept. But the same word referring to both proves that the concept of race is a product of language — and is therefore cultural. The second usage does not only contradict the first usage, but it defeats it.

The upshot is that to decide a case such as Jones’s, we need to know more about how the CMS hair policy is being conceived and used. A general ban on a hairstyle that in the US is primarily associated with African-Americans seems highly suspicious. Maybe the company could prove that it is really not relying on racial cultural stereotypes. But if it cannot, the outright ban should have been considered racially discriminatory. Racism, like race, is not in our DNA; it is in our minds.

• Noah Feldman, a Bloomberg View columnist, is a professor of constitutional and international law at Harvard