Why do unrelated people call each other “bro”?

While it makes perfect sense to call your sibling or cousin “bro” or “brother”, why do we sometimes use this label for friends, acquaintances and even strangers?


After all, it would be weird if someone called a stranger “dad” or “mom” but why is calling anyone “bro” okay?


As you might have noticed after years of exposure to the media, especially social media, people all over the globe sometimes call unrelated fellows “bro” whatever be their language or culture.

Inclusive fitness and genetic relatedness

When people hear or use the phrase “survival of the fittest” they tend to think that “fitness” refers only to physical fitness, which isn’t true. In evolutionary terms, fitness simply means the ability of an organism to survive and reproduce. The greater the fitness, the greater is its ability to pass on its genes to the next generation.

Hence, any trait that promotes survival and reproduction contributes to the fitness of an individual.

Other than your health, attractiveness, and youthfulness, your cognitive skills such as creativity and intelligence can also contribute to your fitness.


More often than not in the animal kingdom, everything doesn’t revolve around an individual. Life is harsh and full of dangers and it’s rare that an individual will successfully pass on its genes living alone. If it’s killed or eaten by a predator, its genes will perish forever into oblivion. 

So, a much more effective strategy would be to form close bonds with other organisms that are genetically related to you. 

This way, not only do the chances of your survival increase but also by helping those who’re related to you survive and reproduce, you’re indirectly enabling your own genes to multiply (because your close kin contain some portion of your genes). This is called inclusive fitness. It’s not only about the fitness of an individual but also about the fitness of its close kin.

The greater the genetic relatedness between two people, the more altruistically they tend to behave towards each other. Your parents contain 50% of your genes and so do your siblings. But you contain 100% of your own genes! So you’re genetically related to yourself the most and are likely to help yourself the most.


All else being equal, you’ll behave more altruistically toward your direct siblings than your first cousins (who contain 25% of your genes), more altruistically towards your first cousins than your second cousins, and so on.

Yes, blood is thicker than water and we’re wired to prefer helping our families over everything else. Even people who have strained relationships with their siblings can go to great lengths to ‘make things right’. Even though they might have had a bad past experience with their siblings, forces deep within them keep pulling them toward maintaining good relations with their closest kin.

hyena brotherhood

The point is that all of us contain behavioral programs that make us behave altruistically toward our closest kin.

It’s these evolved psychological mechanisms that you hack into when you call someone “bro” while seeking any kind of help, support or just to establish some brotherly goodwill.

When we hear people calling us “bro” we tend to treat them as we would our real brothers. Our ancient psychological mechanisms get activated and we tend to confer favors upon them.

Brotherhood is such a strong cohesive force that it’s also used to unite large groups of people. People of a particular group be it a nation, region, religion, caste, culture, subculture – anything they identify themselves with- require cohesion and unity amongst themselves.

That’s why they declare, “We’re all brothers” thereby facilitating the much needed cohesive force to keep the group intact, especially in situations where the unity of the group is shaken.
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