Psychological Tests and Intellectual Disabilities

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Various types of standardized psychological tests are used during the assessment of intellectual disabilities (ID, formerly mental retardation). These tests may assess intelligence (IQ), learning abilities, and behavioral skills. A standardized test is uniformly designed and consistently administered. This permits comparisons of individual scores against average scores for the same group. This comparison provides vital information about a person's skills and abilities relative to their peers. Comparisons between group and individual scores should be matched by age, culture, education, and other factors known to affect IQ scores.

Tests of intellectual functioning (IQ)


Tests of intellectual functioning are designed to measure different mental abilities. These tests are commonly called IQ tests. IQ tests measure the following mental abilities:

  • Reasoning
  • Problem solving
  • Abstract thinking
  • Judgment
  • Academic learning
  • Experiential learning

Previously an IQ score of 70 or below was the recommended cutoff score. The DSM-5 (APA, 2013) has de-emphasized specific IQ scores. Nonetheless, an assessment of intellectual functioning remains central to diagnosis. Thus, IQ scores are still very important considerations. Therefore, it's useful to understand what IQ scores mean.

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If everyone took an IQ test there would be a wide variation in scores. This is called a distribution of scores. The average score (the mean) is the score that is the smallest distance away from all the other scores in the distribution.

When naturally occurring attributes such as IQ (or hat size, for that matter) are measured, the distribution looks like an inverted 'U' shape curve. This curve is called the normal curve. Normal curves have special properties. The area under the normal curve represents the chances of observing a certain score. Then we know how common or uncommon a certain score is.

IQ test scores are standardized. Standardized tests allow a person's score to be compared to other people. IQ tests have a mean score of 100 and a standard deviation of 15. Standard deviation can be understood as the shape of a curve. Some U shaped curves are low, flat, and wide. Other U shaped curves are high, tight, and narrow. Mathematically, standard deviation is a specific distance from the mean. Standard deviation describes this distance and thus the shape of the curve.

Because of mathematical qualities of normal curves, we know that 68% of the area under the normal curve is one standard deviation above and below the mean. Therefore, IQ scores falling between 85 and 115 are common. About 68% of the population would score in that range. About 95% percent of the area under the normal curve occurs between two standard deviations. Therefore, IQ scores falling between 70 and 130 occur 95% of the time. There is a remaining 5%. Half or 2.5% will be above a score of 130. The other 2.5% will be below a score of 70. Therefore, we would expect that only 2.5% of the population would score above 130. This group represents people with an unusually high IQ. They are often termed, "gifted." We would also expect that only 2.5% of the population would score below 70. Historically, scores below 70 indicated an intellectual disability.

IQ tests have two parts. One part measures verbal abilities. The other part measures spatial abilities. Spatial is sometimes called performance skills. It refers to movement and the manipulation of three-dimensional space. Verbal and spatial scores vary widely. One person might have low scores across the board. Another person might do well on verbal IQ but poorly on spatial IQ. Therefore, even though two people have the same total IQ scores, their abilities may be very different. Worried about your mental health? Take our online mental health quiz and get the support you need.

Beyond the Score: Factors Influencing IQ Testing

The interpretation of IQ test results is nuanced and must consider a variety of socio-economic, educational, and cultural factors. It is well-documented that an individual's environment plays a significant role in their cognitive development and, consequently, their performance on IQ tests. For instance, access to quality education, nutritional health, and exposure to enriched environments can significantly impact cognitive abilities. Similarly, cultural biases inherent in test design can disadvantage individuals from different cultural backgrounds, potentially skewing results. To ensure fairness and mitigate biases, it's crucial for test administrators to employ a diverse set of assessment tools and to be mindful of the cultural and socio-economic context of the individuals being tested. Strategies may include using tests that have been validated for specific populations, integrating assessments of adaptive behavior, and considering the individual's background and experiences in the interpretation of results.

Ethical and Responsible Use of IQ Tests

The ethical considerations surrounding the administration of IQ tests are paramount. These tests can significantly influence an individual's educational trajectory and employment opportunities, making it essential to approach testing and result interpretation with care and responsibility. Ethical use of IQ tests involves ensuring that tests are administered by qualified professionals, that individuals are informed about the nature and purpose of the assessment, and that privacy and confidentiality are maintained.

It's vital to consider the potential impact of labeling and to use test results as a guide for supportive interventions rather than as deterministic assessments of ability. The ultimate goal should be to use IQ testing as a tool to identify areas where individuals may benefit from additional support and to foster environments that allow every person to reach their full potential.

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