For Singer (1979/2011), “speciesism” has a specific meaning. It denotes discrimination based on species membership when there is a moral equivalence between individuals in relation to suffering—for example, causing a horse and a person an equivalent amount of pain. His conception designates the prejudicial way in which humans tend to relate to nonhuman animals wherein we often fail to apply a “principle of equality”, specifically, “the principle of equal consideration of interests” (Singer, 1979/2011). In this framework, speciesism occurs when people fail to recognise a moral equivalence between humans and animals when it comes to basic interests. Singer sometimes frames this species-level inequality with analogies made to other forms of prejudice like racism and sexism. Most important is Singer’s emphasis on (in)equality. Arguably, it is what makes Singer’s claims about speciesism rationally persuasive. As such, I believe it is what psychologists should be aiming to study as a psychological construct.

Experimentally, there has been a few attempts to study speciesism in the Singerian sense. Most effectively, Caviola et al. (2022) established that people often exhibit a pro-human bias under conditions where equivalences between human and nonhuman targets are experimentally manipulated—for example, saving a chimpanzee vs. human of higher or lower mental capacity. Psychometrically, however, our field has been less careful about retaining the equality principle. The most widely used Speciesism Scale (Caviola et al., 2019), with over 625 Google Scholar citations to date, in its final form, contains six items. Five of the items focus on approval of specific acts of animal exploitation (e.g., “It is morally acceptable to keep animals in circuses for human entertainment”) or general approval (e.g., “Humans have the right to use animals however they want to”). Only one item conveys something like an equality judgment: “Morally, animals always count for less than humans” (presumably, ‘always’ includes ‘under conditions of equivalent interests’). Other speciesism measures suffer from the same issue. Dhont et al.’s (2016) speciesism scale similarly contains mostly items that assess support for animal exploitation (e.g., “Breeding animals for their skins is a legitimate use of animals”).

Beyond questionable face validity, independent psychometric tests of the Speciesism Scale seem to suggest that it may be measuring something like “general support for animal exploitation”. For example, Suárez-Yera et al. (2021) found that the Speciesism Scale failed to discriminate from a measure of general support for animal uses, the Animal Attitudes Scale (AAS; Herzog et al., 2015), reporting very high inter-scale correlations.

Given the extent of animal suffering at the hands of humans, it is massively important for psychologists to ensure we accurately assess its causes. This means reliably discriminating constructs so that we are not conflating variables where we expect there to be a predictive relationship.

The benefits of conceptual clarity are multiple. By more precisely defining and operationalising speciesism in terms of species-level inequality acceptance, we can study a potential cause of animal exploitation, and this gives us something to aim for when it comes to intervention. It is unlikely that speciesism, in the Singerian sense, is the only cause of animal exploitation. However, when we conflate speciesism and animal exploitation, it is impossible to know how much to attribute exploitation to speciesism vis-à-vis other causal factors, and, therefore, how much attention animal advocates should place on addressing it.

Avoiding psychometric redundancy also helps researchers save precious time and resources. We already have suitable measures of general attitudes towards animal exploitation (e.g., AAS). Having too many of the same measure can create needless noise when comparing across studies, muddying conclusions. Researchers are starting to build psychometric scales to study speciesism in children (McGuire et al., 2025). Modelling these new scales after existing measures could reproduce measurement issues within subdisciplines. Finally, we know that a Singerian notion of speciesism is morally persuasive for many people (Villanueva, 2016). We need to accurately understand how people think about and evaluate moral (in)equalities across species boundaries to better understand why an equality framing can be persuasive.

To reverse this psychometric trend, we need measures that better capture Singer’s concept of speciesism. My colleagues and I tried to do this back in 2015 with items like “When human interests conflict with animal interests, human interests should always be given priority” (see Piazza et al., 2015). Likewise, Dhont and Hodson’s (2014) Human Supremacy Beliefs Scale contains items that appear to capture the equality principle (e.g., “The life of an animal is just not of equal value as the life of a human being”). Items like these get us closer to Singer’s definition. Moving forward, one could imagine items framed as specific (in)equalities between humans and animals or between animal species—e.g., “It is more wrong for fishing companies to kill whales than to kill sharks for human consumption.”

Ultimately, more conceptual clarity around speciesism will allow our field to accurately identify the causes of animal exploitation and better elucidate the prejudice(s) that likely motivate it.