Cause X is a cause area currently neglected by the effective altruism community, typically due to some form of moral blindness or fundamental oversight, yet more important than all the causes currently prioritized by it.

The idea was introduced by William MacAskill,^[1] possibly by analogy with Derek Parfit's "Theory X", a currently unknown hypothetical theory which would solve a number of important open problems in population ethics.^[2] ("Cause X" is also sometimes used loosely to refer to any promising and neglected cause,^[3]^[4] but this is not how the expression is generally understood.)^[5]

Does Cause X exist?

The existence of Cause X should not be taken for granted; whether there is a Cause X is an open question. In contrast to Parfit's Theory X, however, there are no impossibility theorems that could disprove the existence of Cause X.^[6] Furthermore, if there is a Cause X, there is no determinate way of ascertaining that it has been found; this is another disanalogy with Theory X, whose conditions are defined with sufficient precision that it is possible to establish when a theory satisfies them.

There are, however, good reasons for expecting Cause X to exist. All previous generations overlooked highly important causes—usually by neglecting large groups of morally relevant beings, such as women, racial minorities, and nonhuman animals—,^[7] so it would be a remarkable coincidence if our generation was the first to avoid this moral shortcoming.^[1]^[8]^[9] In addition, the appearance that the present generation is different may itself be debunked as a manifestation of the "end of history illusion",^[10] "new era thinking",^[11] and related cognitive biases. Furthermore, the space of possible cause areas is vast, and humans have only recently begun to explore it systematically, so it seems antecedently very likely that a cause more important than the current top causes remains to be found.^[8] Yet another argument for the existence of Cause X is that the effective altruism community has, to a certain extent, changed its views about which causes are most important over the years. An induction from this history suggests further changes in what causes the community will consider most impactful.

Heuristics for finding Cause X

A number of heuristics for finding Cause X have been proposed.^[3]^[12] Kerry Vaughan suggests three ...

The climate change policy with the most potential is the most neglected

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