Why do we buy a Toyota Prius but do not take as much care to make our homes more efficient?Don’t misunderstand me, I have nothing but admiration for the Toyota Prius. But let’s look at the numbers. Switching from a standard midsize car to the Prius can reduce CO2 emissions from 7.5 tons to 4.4 tons per year (a 3.1 savings).But consider this: A standard four-bedroom house occupied by four people in Massachusetts can produce 53 tons of CO2 a year. What if we took steps to make the home’s heating and cooling system more efficient, installed efficient lighting, used ENERGY STAR appliances, and took steps to reduce energy used for hot water? If we made all these changes, the same house could produce 30 tons of CO2 a year (savings of 23 tons).So why do we buy and proudly drive the Prius but do not spend nearly as much on making our home more efficient? I think that there are at least three reasons. The first is that we remember the price of gas at the pump, and seeing the gas price every time we fuel-up makes it more psychologically painful to drive a less efficient car. The second is that when we drive a Prius it acts as a constant reminder of how little we spend on gas compared to all the cars driving around us–providing us with positive comparison. The third is that we project a positive social image when other people see us driving a socially conscious car.Homes don’t have any of these features. We don’t remember the price of heating, lighting, etc. We don’t see other people and think about how much they are paying to heat and cool their homes, and we don’t get any social bonus points for making our homes more efficient.What is the point of all of this? I think we can look at the success of the Prius, and by understanding the underpinning of its success, we can design incentives so that people start caring about making their homes more efficient.