One end of the normal distribution of intelligence scores is defined by people with very low IQ. Intellectual disability is the term used to describe those individuals  who have an IQ below 70, who have experienced deficits since childhood, and who have trouble with basic life skills, such as dressing and feeding themselves and communicating with others (Switzky & Greenspan, 2006). About 1% of the Canadian population, most of them males, fulfill the criteria for intellectual disability, but some children who are diagnosed as mentally disabled lose the classification as they get older and better learn to function in society. People with low IQ are particularly vulnerable to being taken advantage of by others; this is an important aspect of the definition of intellectual disability (Greenspan, Loughlin, & Black, 2001). Intellectual disability is divided into four categories: mild, moderate, severe, and profound. These are not quantifiable categories; they are described in qualitative terms based on the level of impact they have on daily functioning. Severe and profound intellectual disabilities are usually caused by genetic mutations or accidents during birth, whereas mild forms have both genetic and environmental influences.

One cause of intellectual disability is Down syndrome, a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. The incidence of Down syndrome is estimated at one per 800 to 1,000 births, although its prevalence rises sharply in those born to older mothers (Figure IL.8). People with Down syndrome typically exhibit a distinctive pattern of physical features, including a flat nose, upwardly slanted eyes, a protruding tongue, and a short neck.

In the first half of the 20th century, many psychologists, along with huge numbers of other influential people in society, including President Roosevelt, embraced the horrific field of eugenics. The eugenics movement was founded by an English scientist called Francis Galton, who misappropriated Charles Darwin’s work on evolution and Gregor Mendel’s work on heritable traits in plants and animals, by arguing that these ideas could be extended to human traits. Eugenicists believed that genes, not environment, determined intelligence (among other traits), and that people with low intelligence should be prevented from “spreading their genes”. These beliefs were extremely widespread across Europe and the United States (US). Many of the Western psychologists involved in developing intelligence tests were eugenicists, including Spearman in the UK, and Terman in the US. As a result, intelligence testing was weaponized as evidence of low intelligence among people with little formal education. This included women, people of colour, and people living in poverty (Guthrie, 1998). Eugenics was used to promote and justify social and economic inequalities in the US, the United Kingdom, and the countries that they colonized. The long-term consequences of eugenics-inspired oppressive socioeconomic policies still persist today.

In addition to his eugenicist beliefs, Spearman (1904) also popularised the idea that intelligence could be adequately measured by a single score that represented a person’s general intelligence (Boake, 2002; Guthrie, 1998). Spearman justified this approach because he had found that people who were proficient in one intellectual domain, e.g., verbal skills, were often equally adept in other domains, such as quantitative reasoning (Biswas-Diener, 2018; Guthrie, 1998).

In the 1940s, Raymond Cattell proposed that intelligence consists of two major factors: crystallised intelligence and fluid intelligence (Cattell, 1963). Crystallised intelligence is characterised as the ability to retrieve knowledge related to your educational and life experiences. When you learn, remember, and recall information in your classes, you are using crystallised intelligence. Fluid intelligence on the other hand, encompasses the ability to cope with new situations, see complex relationships, and solve abstract problems (Cattell, 1963). More recent theories suggest that intelligence consists of multiple different domains that go beyond cognitive skills; a person may be strong in one, but not others (Gardner, 1983; Sternberg 1999). The first widely used intelligence test in the US was designed to measure general intelligence, and so minimised the strengths of people who were not good “all-rounders”. The test was developed in 1916 by Lewis Terman, a eugenicist and a professor at Stanford University. Terman adapted the Binet-Simon Scale for use in the US and renamed it the Stanford-Binet scale. Scores on the Stanford-Binet scale were referred to as the intelligence quotient or IQ (Guthrie, 1998). Terman standardised the Stanford-Binet scale using data from 1000 children and 400 adults in the US (Guthrie, 1998), all of whom were White.

Standardisation is a commonly used procedure when developing psychological tests. Raw scores are converted so that they fall in a normal distribution, which is shaped like a “bell curve” to allow for easy comparisons between individuals (Biswas-Diener, 2018). It is therefore important that the scores are standardised using a large sample of people who are representative of the population. In the Stanford-Binet (and other subsequent IQ tests) the mean IQ score is set at 100. In the standardised sample, 68% of people have an IQ between 85 and 115, and 96% of people have an IQ of between 70 and 130. There are very few people at the extremes of the scale. You probably have taken at least one standardised aptitude test (like the SAT) in your life. The scores on these tests are standardised in a similar way, but the average score is set around 500 on each SAT subtest.

Excluding people of colour from the standardisation process was a serious flaw in Terman’s Stanford-Binet intelligence test, as the standardised scores were not representative of the population (Guthrie, 1998). The test relied heavily on verbal skills and asked questions that reflected middle-class values, so children with little schooling were often misclassified as having low intelligence. The same issues that made the Stanford-Binet test culturally unfair for less educated children also plagued the adult intelligence tests that were developed for the army during the First World War (Guthrie, 1998).

In 1917, Yerkes (another eugenicist), Terman, and other psychologists from the American Psychological Association helped to design intelligence tests for the US army. These were used to determine work assignments for army recruits (Guthrie, 1998). Not only were the tests culturally biased, but the military psychologists who administered them were all White men; many of them were blatantly racist, which probably influenced their scoring (Guthrie, 1998). The army then released reports based on the test scores, stating that non-Whites, Jews, and Eastern and Southern European immigrants were intellectually inferior to Whites. Moreover, they reported that people with darker skins were less intelligent than light-skinned people (Guthrie, 1998). This misguided report was widely disseminated and was used to further justify and promote many harmful socioeconomic policies. In the US, interracial marriages were banned, and racial segregation was strongly enforced, to decrease the potential development of inter-racial relationships. Policies such as redlining (a discriminatory practice in which financial services are withheld from neighbourhoods that have significant numbers of racial and ethnic minorities) and racially segregating schools, increased wealth and resources for Whites and reduced economic opportunities and educational resources for people of colour. The 1924 Immigration law severely restricted the admittance of Jews, Eastern and Southern Europeans, and people who were mentally ill, to the United States (Guthrie, 1998).

In keeping with eugenicist ideals, by 1944 40,000 people had been subjected to forced sterilisation in the United States, and 30 states had passed laws legalising this practice. Forced sterilisation targeted mostly poor Black women and immigrants and included people living in institutions for the mentally ill, people accused of crimes, and people with disabilities or chronic illnesses. Scandinavia and Canada also embraced forced sterilisation during the same general time period (Kevles, 1999). Adolf Hitler and other German eugenicists were inspired by the widespread ways that the US embraced White supremacy through its sterilisation and immigration laws. However, the policies that Germany subsequently created were even more extreme; they mandated killing young children with physical or mental disabilities. Eugenics was also used to justify the persecution and mass murders of six million Jews during the Holocaust (Guthrie, 1998). Sterilisations in the United States declined dramatically in the aftermath of the atrocities of the Holocaust and in the light of new studies showing that intelligence and mental illness are not solely determined by genes. However, even as laws began to change, the US still carried out about 22,000 more forced sterilisations across 27 states between 1943 and 1963. Some of these sterilisations occurred in some southern and mid-Western states shortly after World War II, when reproductive clinics began offering sterilisation as a method of birth control to young, mostly Black, poor women in rural areas, and to young native American women. Many of these sterilisations were coerced or done without the woman’s consent (Reilly, 2015). It was not until the 60s and 70s that US sterilization laws were largely repealed (Sofair & Kaldjian, 2000; Suarez-Balcazar, 2022). However, reproductive injustice continues even today. In 2020, a nurse working for Immigration and Customs Enforcement (ICE) made it publicly known that several Latina women were coerced into having unnecessary hysterectomies during their detention by ICE (Suarez-Balcazar, 2022).

Eugenics became popular in Canada and the United States in the early 20th century and was supported by many prominent psychologists, including Sir Francis Galton (1822–1911). Dozens of universities offered courses in eugenics, and the topic was presented in most high school and university biology texts (Selden, 1999). Belief in the policies of eugenics led the Canadian legislatures in Alberta (1928) and British Columbia (1933) to pass sterilisation laws that led to the forced sterilisation of approximately 5000 people deemed to be “unfit.” Those affected were mainly youth and women, and people of Indigenous or Metis identity were disproportionately targeted. This practice continued for several decades before being completely abandoned and the laws repealed. This dark chapter in Canadian history shows how intelligence testing has been used to justify political and racist ideologies.