Key takeaways UCLA researchers grew brain cells from dozens of genetically distinct donors together in a cell village, then built a statistical tool called Townlet to measure each donor's cell fitness — how well their cells grow and survive. Why cell villages beat separate dishes for spotting genetic differencesCultured in separate wells, cells are subject to tiny variations in oxygen, temperature and handling that can bury subtle but important biological differences. To measure fitness, the team tracked what share of the village each donor's cells claimed over time, using DNA sequencing as a running census. "The readout is proportional data — essentially a pie chart changing over time," said co-senior author Harold Pimentel, assistant professor of computational medicine, computer science and human genetics at the Geffen School of Medicine. That points to early brain cell overgrowth as a key driver of the larger heads seen in some people with autism.