That’s according to the chartered psychologist Catherine Hallissey, who says that, while these associations can be helpful, they can also cause us to fall into the trap of stereotype and bias. “Categories – including the ones we place ourselves in – are shortcuts to thinking and reasoning about the social behaviour of others,” Hallissey explains.

“The benefit of categorisation is that it helps us simplify our social worlds so we can respond to new experiences quickly and efficiently. But the potential downside is that we view others in the same category as us, the ‘in group’, more favourably than those in other categories, the ‘out group’, which is the foundation of bias, stereotyping and prejudice. These shortcuts to organising your social world mean you may also overlook the uniqueness of each person.”

In this way, while being categorised as the pink or purple sister may not have the power to influence the way we see ourselves irreversibly, these associations can send us subconscious messages. For example, Hallissey explains, when siblings are categorised as ‘the sporty one’, ‘the smart one’ or ‘the arty one’, these categories can quickly become all-consuming – leaving little space for each person to explore aspects of their identity that fall outside of these categories.

The surprising psychology behind the ‘pink or purple sister’ phenomenon going viral on TikTok