Why does “us vs. them” show up so reliably in everyday language? In two online experiments (**N = 922**), we measured the *emotional positivity* (valence) of the words people used to describe an in-group vs. an out-group—using a large psycholinguistic database to score how positive/negative each word is. We found a consistent **in-group positivity boost**, especially when the input information about the group was negative, and we showed that this positivity bias can **propagate across ‘cultural generations’** in a diffusion-chain experiment. This project was published in [Nature: Scientific Reports](https://www.nature.com/articles/s41598-024-58905-y).