At the time of the studies, research exploring the self-administration of morphine in animals often used small, solitary metal cages. Alexander hypothesized that these conditions may be responsible for exacerbating self-administration. To test this hypothesis, Alexander and his colleagues built Rat Park, a large housing colony 200 times the floor area of a standard laboratory cage. There were 16–20 rats of both sexes in residence, food, balls and wheels for play, and enough space for mating. The results of the experiment appeared to support his hypothesis that housing-conditions affect the consumption of morphine water. This research highlighted an important issue in the design of morphine-self administration studies of the time, namely the use of austere housing-conditions, which confound the results.