Year One: 1964
In the first year of publication, Paul et al. (1963) gained sixteen citations. Most of these citations only cite Paul et al. (1963) (n = 12) but four of them cite both Paul et al. (1963) as well as one of its citing papers.
Citations that don't include Paul et al. (1963) are restricted to their own topic, at this point in time. This means, for example, that caffeine papers cite caffeine papers or physical activity papers cite physical activity papers. There is no connection between groups, meaning that a physical activity paper does not cite a caffeine paper.