Laboratory studies provide additional information on the causal relationship between motivation and marijuana. The Mendelson experiment, where hospitalised volunteers worked on an operant task to earn money and marijuana for 26 days, found that the dose of marijuana smoked did not influence the amount of work done by either the casual-user group or the heavy-user group; all remained motivated to earn and take home a significant amount of money in addition to the work they did for the marijuana. It seems clear that marijuana does not cause a loss of motivation.