Two groups of eggs were set 12 hr. apart. Median hatching ages were separated by the same temporal duration and it was assumed that developmental age would be isomorphic with the time-of-day-at-test. Groups of chicks 3, 9, 15, and 24 hr. of age were given a choice between age-mate and hen; the results indicate that age-since-hatching and time-of-day-at-test are both variables in released social responses. Further analysis of the data shows that the trial effect is a product of changing preferences, not a difference in “arousal,” or sleep-wakefulness.
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