I've recently been reading about the psychology of emotions, and one paper (Geng et al. 2013) struck me as relevant to the question of when to judge a zoo visit. I've cited it below, but in short the paper explores a phenomenon known as the peak-end rule. This states that memories of emotional experiences are primarily based on how the person felt at the peak intensity and end of the experience, rather than an average of every moment. Empirical research on the peak-end rule has given mixed results and Geng et al. offer two reasons for this: 1) Different studies use different retention intervals. In particular, Geng et al. argue that the peak-end rule holds when the experience was relatively recent, but an average of the whole experience is taken when more distant memories are recalled. 2) People use different information about an experience to recall their emotional response. Episodic memory is tied to particular experiences or snapshots (in the case of the peak-end rule, the peak and the end), whereas semantic memory is based on generalizations or averages of the whole experience. Geng et al. performed two experiments to show that memories of recent events are episodic (follow the peak-end rule), but memories of more distant events are semantic (a generalisation of the whole experience). The "watershed" between long and short retention intervals is 3-7 weeks. So, here's the point: providing zoo visits elicit an emotional response (and why would we love zoos if they didn't?), our initial post-visit evaluations are likely to reflect only the peak intensity of emotion (perhaps the best/worst exhibit) and the end of the trip (or last exhibit). According to Geng et al., we would evaluate zoos more holistically if we instead left our judgements for a month or two afterwards. Only then will we take the whole experience into account. Geng, X., Chen, Z., Lam, W., & Zheng, Q. (2013). Hedonic evaluation over short and long retention intervals: The mechanism of the peak–end rule. Journal of Behavioral Decision Making, 26(3), 225-236.