We study Drosophila to understand the interplay between genes, individual behaviour and the environment. How do we do this? The circadian timing system (a daily clock) has several inherited features that include input pathways for sensing time cues, oscillators for keeping time, and output pathways for generating "rhythms." We study how flies keep time and how they share time cues with each other. Current data suggest that flies send out and receive time cues using chemical signals. We would like to understand this. This research about social influences on circadian rhythms in Drosophila shows one example of how groups affect individual behaviour and, perhaps, gene expression. We also plan to develop other examples towards defining the principles that relate group dynamics, individual behaviour and gene expression in humans.