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 (MacAskill 2016), 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 (Parfit 1984). ("Cause X" is also sometimes used loosely to refer to any promising and neglected cause (Savoie 2019; Gómez Emilsson 2019), but this is not how the expression is generally understood (Rice 2019).)

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 (Ng 1989). 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 (Karnofsky 2017)—, so it would be a remarkable coincidence if our generation was the first to avoid this moral shortcoming (Williams 2015; MacAskill 2016; cf. Lewis 2015). In addition, the appearance that the present generation is different may itself be debunked as a manifestation of the "end of history illusion" (Quoidbach, Gilbert & Wilson 2013), "new era thinking" (Schiller 2015), 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 (Williams 2015). 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 (Vaughan 2016; Savoie 2019). Kerry Vaughan suggests three such heuristics:

Heuristic 1: Moral circle expansion. One apparently robust historical trend since at least the last few centuries is the gradual expansion of the circle of moral concern (Singer 1981; Pinker 2011; but cf. Branwen 2019). Furthermore, this expansion appears to account for much of the moral progress that occurred during this period. Thus, a plausible heuristic is to push this expansion even further. This heuristic suggests wild animal welfare, invertebrate welfare, digital sentience, as well as research on moral patienthood, as Cause X candidates.

Heuristic 2: Transformative technology. Technological progress has the potential to radically change the world in moral relevant respects. This assessment is plausible both on the basis of historical analysis—many of the most significant changes have occurred due to some form of human innovation—and upon consideration of various anticipated technologies not yet developed, such as whole brain emulation,  artificial general intelligence, and atomically precise manufacturing. The heuristic suggests work on making these technologies safer, such as AI safety, as well as differential technological development, as Cause X candidates.

Heuristic 3: Crucial considerations. A crucial consideration is one that warrants a major reassessment of a cause's impact. Actively looking for such considerations is thus of clear relevance for finding Cause X. Here the heuristic would favor making lists of crucial considerations, as well as the search for additional "deliberation ladders" in existing arguments for specific causes.

The meta-heuristic of holding events where attempts are made to find potential Cause X candidates has also been suggested as an effective discovery method (Gómez Emilsson 2020). Such events could take the form of informal meetups, conference workshops, or academic conferences.