Generally, junior professors write long and unsolicited emails to senior professors, who reply with short ones after a delay; the juniors then reply quickly and at length. This is not because the seniors are busier, for they, too, write longer and more punctually when addressing their deans and funders, who reply more briefly and tardily. The asymmetry in length and speed of reply correlates with dominance.
When a subordinate chimpanzee grooms a dominant one, it often does so for a long time and unsolicited. When it then requests to be groomed in turn, it receives only a brief grooming and usually after having to ask a second time.
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