How Many Faces Do We Know?
A 2018 study by Jenkins et al. estimated that the average person can recognise approximately 5,000 faces, a number that surprised many researchers. The study measured this by showing participants large numbers of celebrity and personal contact faces and counting distinct identities they could recognise. Individual variation was large, with ranges spanning from around 1,000 to over 10,000.
This database is built over a lifetime of social exposure. Face memories are more durable than most other types of memory, people often recognise classmates they have not seen in 30+ years from a single photo. The face memory system is specifically optimised for long-duration storage and rapid retrieval.
How the Brain Indexes Faces
The brain's face memory system stores face representations in a way that differs from other object memories. Faces are stored as multi-dimensional representations in distributed memory structures, with connections to associated information (name, context, personal relationship). The same face can be recognised from a single feature (distinctive nose) or from the holistic gestalt without any individual feature being diagnostic.
The multi-dimensional storage is conceptually similar to embedding space: each stored face is a point in a representational space where proximity reflects similarity. Recognition occurs when a new perceptual experience activates the stored representation most strongly, the brain is performing something analogous to nearest-neighbour search, just implemented in biological neural tissue.