Trace conditioning frequency and magnitude (inset) to shock-shock pairings develops as a function of age. Mean (± SEM) percent conditioned responding (%CRs) to the shock-alone unconditioned stimulus (US1) trials for rat pups given paired (Paired) or explicitly unpaired (Unpaired) US1 and US2 presentations beginning at postnatal day 12, 15, and 18 (P12, P15, and P18) across three sessions of stimulus presentations on Day 1 and three sessions of stimulus presentations on Day 2. The inset shows mean conditioned response (CR) magnitude to the shock-alone unconditioned stimulus (US1) trials for rat pups given Paired or explicitly Unpaired US1 and US2 presentations beginning at postnatal day 12, 15, and 18 (P12, P15, and P18) collapsed across sessions and days.