The market demand curve of a good that is rival in consumption and excludable is the horizontal sum of the individual demand curves. However, the marginal social surplus curve of a public good is the vertical sum of the individual marginal benefit curves. What aspect of a public good causes the demand curves to be summed vertically instead of horizontally? Question 10 options: Whether we sum vertically or horizontally depends only on whether the good is excludable. Whether we sum vertically or horizontally depends only on whether the good is rival in consumption. A vertical sum of individual demand curves is the same thing as a horizontal sum of individual demand curves. Whether we sum vertically or horizontally depends on whether the government, as opposed to the free market, usually provides the good