Accents
Katherine Kinzler has demonstrated that preverbal infants as young as 5-6 months of age “prefer” their own native speakers. She found that American infants look longer at someone speaking with an American accent than someone with a French accent, and the opposite pattern occurs with French infants. And when two adults simultaneously offer a 10-month-old the same toy, the baby usually reaches for the one being given to them by the native speaker.
In the ancestral past, neighboring communities were often at war with each other, and the most reliable marker of an out-group member wasn’t what they looked like but how they sounded.