For example, the gradual shift in the United States towards smaller vehicles, which began after the 1974 oil shock, went into reverse as cars got more fuel efficient. Disentangling these effects is complex because lower oil prices contributed to this result, but the rebound effect appears to be a factor. Somewhat different implications arise when the energy is being consumed by a business: it means more output per unit of energy' consumed, whereas for households it may mean increased final consumption of heat or electricity, and hence an increase in welfare.