I dug up an old paper that is relevant to the point Rhys Southan of Let Them Eat Meat brought up in a recent post which I quoted on June 9:

But I recognize that my consumer choices are almost totally insignificant in this regard; like veganism, this is a symbolic gesture.

The paper is Expected Utility, Contributory Causation, and Vegetarianism by Gaverick Matheny (Journal of Applied Philosophy, Vol. 19, No. 3, 2002. p.293-7.). In it, Matheny argues that act-utilitarians cannot know what the actual consequences of an action will be in many cases, and therefore they must base their actions on the probability of expected consequences. Matheny goes on to explain that by not eating meat for a year, you take a chance that you will be the person who causes a reduction of meat past a threshold which is felt to the farmers and causes them to raise less animals:

For example, take the case of The 200 Million Consumers. There are 200 million consumers, each of whom eats 50 farm animals each year. In this market there are only ten possible annual outputs of animals for farmers: one billion animals, two billion, and so on, up to ten billion. The difference between each of these annual outputs, one billion, is the smallest unit of demand perceivable to the farmer and is thus the threshold unit. Since there are 20 million customers per threshold unit, and only one of these customers will actually complete the unit of which his or her purchase is a part, the probability of my completing a unit is one in 20 million. That means by buying meat I have a one-in-20 million chance of affecting the production and slaughter of one billion animals. The expected disutility is then one-20-millionth times one billion, which equals 50 – that is, the disutility associated with raising and slaughtering 50 animals per year.

Matheny explains why this matters using the example of The 100 Bandits in which 100 bandits go into a village and each steals one bean from each of 100 villagers, each of whom has 100 beans. After this is done, the villagers have no beans left. However, the loss of only one bean cannot cause any perceptible difference to a villager. Since no bandit could have caused actual (perceptible) harm by stealing only one bean, none is responsible for the villagers going hungry, right?

No. One of the bandits stole the nth bean that reached the threshold of perceptible harm to a villager, and the probability of any given bandit being the one who steals the nth bean to reach that threshold is the same whether each bandit steals 100 beans from the same villager or 1 bean from 100 different villagers.

In other words, when divided into equal contribution units, any contribution of a unit towards reaching the threshold of a perceptible difference is as morally important as the unit that actually reaches that threshold.