On many issues, I find that people hold the following two views:

  • If many people did this thing, then change would happen.
  • But any individual person doesn’t make a difference.

Holding that combination of views is usually a mistake when we consider expected value.

Consider ethical consumption, like switching to fair-trade coffee, or reducing how much meat you buy. Suppose someone stops buying chicken breasts, instead choosing vegetarian options, in order to reduce the amount of animal suffering on factory farms. Does that person make a difference? You might think not. If one person decides against buying chicken breast one day but the rest of the meat eaters on the planet continue to buy chicken, how could that possibly affect how many chickens are killed for human consumption? When a supermarket decides how much chicken to buy, they don’t care that one fewer breast was purchased on a given day. However, if thousands or millions of people stopped buying chicken breasts, the number of chickens raised for food would decrease—supply would fall to meet demand. But then we’re left with a paradox: individuals can’t make a difference, but millions of individuals do. But the actions of millions of people are just the sum of the actions of many individual people. Moreover, an iron law of economics is that, in a well-functioning market, if demand for a product decreases, the quantity of the product that’s supplied decreases. How, then, can we reconcile these thoughts?

The answer lies with expected value. If you decline to buy some chicken breast, then most of the time you’ll make no difference: the supermarket will buy the same amount of chicken in the future. Sometimes, however, you will make a difference. Occasionally, the manager of the store will assess the number of chicken breasts bought by consumers and decide to decrease their intake of stock, even though they wouldn’t have done so had the number of chicken breasts bought been one higher. (Perhaps they follow a rule like: “If fewer than five thousand chicken breasts were bought this month, decrease stock intake.”) And when that manager does decide to decrease their stock intake, they will decrease stock by a large amount. Perhaps your decision against purchasing chicken breast will have an effect on the supermarket only one in a thousand times, but in that one time, the store manager will decide to purchase approximately one thousand fewer chicken breasts.

This isn’t just a theoretical argument. Economists have studied this issue [Norwood & Lusk 2011, p. 223] and worked out how, on average, a consumer affects the number of animal products supplied by declining to buy that product. They estimate that, on average, if you give up one egg, total production ultimately falls by 0.91 eggs; if you give up one gallon of milk, total production falls by 0.56 gallons. Other products are somewhere in between: economists estimate that if you give up one pound of beef, beef production falls by 0.68 pounds; if you give up one pound of pork, production ultimately falls by 0.74 pounds; if you give up one pound of chicken, production ultimately falls by 0.76 pounds.

This same reasoning can be applied when considering the value of participating in political rallies. Suppose there’s some policy that a group of people want to see implemented. Suppose everyone agrees that if no one attends a rally on this policy, the policy won’t go through, but if one million people show up, the policy will go through. What difference do you make by showing up at this rally? You’re just one body among thousands of others—surely the difference you make is negligible. Again, the solution is to think in terms of expected value. The chance of you being the person who makes the difference is very small, but if you do make the difference, it will be very large indeed. This isn’t just a speculative model. Professors of political science at Harvard and Stockholm Universities analyzed Tea Party rallies held on Tax Day, April 15, 2009 [Madestam, Shoag, Veuger & Yanagizawa-Drott 2013]. They used the weather in different constituencies as a natural experiment: if the weather was bad on the day of a rally, fewer people would show up. This allowed them to assess whether increased numbers of people at a rally made a difference to how influential the rally was. They found that policy was significantly influenced by those rallies that attracted more people, and that the larger the rally, the greater the degree to which those protestors’ representatives in Congress voted conservatively.

William Macaskill, Doing Good Better, 2015

Added to diary 15 April 2018