Which of the following correctly describes the relationship between intelligence and iq?

Intelligence

The intelligence quotient (IQ) is often reported in studies of SDB although findings are not consistent. Lower IQ scores have been reported in children with SDB compared to controls, although these scores are typically still within the normal range.19,21,49,50,55,59,92–96 One study in children awaiting adenotonsillectomy found that compared to healthy non-snoring children, the snoring children had a 10-point reduction in IQ.95 Of course, the clinical significance of this remains to be shown for a high-functioning child, but a 10-point IQ difference could be rather significant in children performing at a lower level. Several studies fail to support findings of differences in full-scale IQ,44,91,97,98 although some have found lower scores for verbal IQ (language skills) in children with SDB.44,97

Lack of robust findings between SDB and IQ is perhaps not surprising given that measurement of IQ is complex and, in essence, measures performance across several tasks rather than a focus on a particular area of cognition. Standardized vocabulary tests, as a proxy measure of IQ and an excellent predictor of cognition and academic success,99 have demonstrated that the difference in scores between children with and without SDB may be equivalent to the impact of lead exposure.100 These findings clearly have great clinical significance for a child's future if indeed they are supported by additional studies. Nonetheless, there are many other factors which clearly impact a child's IQ and which require consideration in studies of SDB, including genetics, parental education level, as well as biological and environmental factors.23,101,102

The intelligence quotient

Intelligence quotient (IQ) refers to mental age (MA) expressed as a ratio of chronological age (CA) multiplied by 100. For IQ to remain stable, MA must increase with CA over time. This is true until around 18 years, when intellectual abilities are usually fully developed. As stated above, it was erroneously thought that intellectual ability started to decline after young adulthood. It now appears that although fluid intelligence may start to decrease, crystallized intelligence continues to develop throughout the lifespan. Intelligence is said to be normally distributed with a slight ‘bump’ at the lower end of the normal distribution curve, representing those with severe learning difficulties.

Issues in Summarizing Overall Ability

The FSIQ has strong explanatory and predictive power at the group and individual level. Still, the use of an overall summary score may mask individual differences among the broad domains of general ability, especially in patients with neuropsychological deficits where the focus of clinical attention is not prediction but diagnosis of underlying cognitive deficits. For this reason, the relevance of reporting IQ scores has been questioned (Fiorello et al., 2007). Yet, other researchers suggest that FSIQ may be an equally valid measure of general ability for individuals or groups having highly variable index scores as for those having consistent index scores (Daniel, 2007) and that there may be no difference in the predictive validity of FSIQ for low-scatter and high-scatter groups (Watkins, Glutting, & Lei, 2007).

FSIQ is an especially strong predictor of educational attainment, occupational level, memory functioning, and school achievement (Wechsler, 2008, 2009). FSIQ and achievement, for example, correlate strongly, typically around 0.70. This means that FSIQ explains about half the variance in achievement. There is no known variable or collection of variables that comes close to fully accounting for the other half. Beyond the relationship with achievement, there is considerable ecological and criterion validity for the use of an overall estimate of general intelligence in a variety of areas related to success in life including pre-employment testing and predicting job performance (Gottfredson, 1997, 1998; Kuncel, Hezlett, & Ones, 2004). Thus, when the focus of clinical attention is prediction then FSIQ is often the most potent predictor. When the focus is clinical diagnosis of pathology then the index scores are often more informative.

Unlike every other Wechsler interpretative system that has been written, we discuss FSIQ last, rather than first. This is not to devalue the explanatory and predictive power of the FSIQ, but to emphasize the descriptive clinical power of the WAIS–IV Indexes and to place FSIQ in its proper role of providing a relevant backdrop against which the index scores are evaluated. This is consistent with our position that the first line of clinical interpretation rests with the index scores.

Publisher Summary

Intelligence quotient (IQ) can be obtained by the equation MA/CA=IQ, where MA is mental age and CA is chronological age. This chapter discusses the concepts of mental age and IQ, and the operations by which they are measured. It is often argued that true intelligence cannot be measured by the best of tests but, if this is true, then the issue is without empirical content and need not concern one from a scientific point of view. Intelligence is defined as what the intelligence test measures. The chapter explains that this is accepted as the first step and discusses what intelligence tests measure and how they measure it. Environmentalists, that is, the established school authorities, were in effect allowed to define intelligence in the successful approach to mental testing. The first practical stratagem was to select items predictive of progress in school. The second practical stratagem devised by the mental testers was the standardization group. They recognized that American school children at the turn of the century formed a motley group with parents of different social, ethnic, and language groupings. The third practical stratagem employed by the mental testers was to write items designed to yield a direct monotonic relationship to increasing age, which is to say, that an item must be passed by progressively larger numbers of older children.

Epidemiology

Using an IQ of less than 70 (2 standard deviations below the mean) as the criterion for learning disability or mental retardation should result in prevalence rates of 2.3% based on the properties of the normal distribution. Studies of mild mental retardation have produced widely varying prevalence estimates, however, from less than 0.5% to over 8%. There are at least several reasons for this variation. First, identification, or ascertainment, of the ‘at risk’ population affects the rate, with studies that survey the entire population producing higher rates than those relying on cases formally identified because of health or educational problems, as many people with IQs between 50 and 70 may not be formally registered. Second, the test administered may affect the rates identified. Because performance on IQ tests has improved over the years (a phenomenon called the Flynn effect), the use of older and ‘easier’ tests may produce a lower rate of learning disability. There may also be real differences in the rate of mental retardation across different populations, with more advantaged populations having lower rates of learning disability. Some of this population difference may be due to bias in test content, so that children from developed countries who attend school from an early age may be more familiar with the type of items on conventional IQ tests. However, use of the supposedly culture-fair test does not eliminate the differences in rates of learning disabilities.

For severe mental retardation, however, the reported prevalence rates have been more consistent and average around 0.4–0.5%, which is about 10 times greater than expected where the normal distribution maintained. This extra ‘hump’ at the bottom of the normal IQ distribution is likely to represent the children whose learning disabilities have a clear ‘organic’ origin whether caused prenatally, perinatally, or postnatally.

These findings have led to a suggestion of a ‘two-group’ approach to learning disability. The first group represents the lower end of a normally distributed population, for which no organic cause can be ascertained and encompasses the majority of children with mild learning disability. Environmental deprivation has classically been cited as the cause of cognitive delay in this group, although this is now criticized for being too simplistic. The second group consists of those children with defined organic or biological cause for their cognitive impairment, which may be more severe. Again, this is likely to be an oversimplification of the heterogeneity of learning disability, and as advances are made in molecular genetics, increasing numbers of children with mild learning disability are likely to be found to have genetic disorders. However, the division of learning disability into these two broad etiological groups has been a useful starting point for researchers in this field.

There is also criticism of the use of IQ as sole discriminator of learning disability for both epidemiological studies and administrative purposes. In terms of judging prevalence rates based on IQ, problems arise due to the fact that IQ may not remain stable throughout development and IQ scores in infancy typically have only low correlations with scores in later childhood. Further, researchers found that although once formal schooling begins IQ scores tend to be more robust, individual cases can still show major gains or losses in cognitive ability. Up to now, however, there has been no agreement on the way in which adaptive behavior should be measured and impairment defined to meet this additional criterion for diagnosis.

The term ‘administrative prevalence’ has been used to mean the numbers for whom services would be required in a community which made provision for all who needed them. Here again, using IQ alone as discriminator is inadequate as IQ scores do not correlate with social adaptation in all instances. Indeed, researchers found that although 2.5% of children in the study could be classified as having learning disability based on their IQ scores, only half of these children were sufficiently impaired in their daily lives to require the additional provision of services. Functional impairment is dependent not only on the child’s developmental level, but on wider social issues of available family and community resources. Thus, it is unsurprising that administrative prevalence is somewhat higher in lower socioeconomic groups.

Gender differences are also apparent in the rates of learning disability with a male to female ratio of 1.5:1, which may reflect the male preponderance in certain genetically mediated disorders affecting cognitive ability. These differences are more clear-cut for those with severe learning disability.

3.4 Cognitive Phenotype

The range of intelligence quotient (IQ) scores seen in CdLS is wide, although most individuals diagnosed with CdLS do have an ID (Hawley, Jackson, & Kurnit, 1985). Oliver et al. (2008), using the Vineland Adaptive Behavior Scales, reported that 50% of individuals have a profound ID; the proportions were 24%, 15% and 11% for severe, moderate and mild ID, respectively. Formal IQ testing has shown that the mean IQ score of people with CdLS falls within the moderate ID range (Kline et al., 1993). Strengths in the areas of perceptual organization and visuospatial memory have been described, but this warrants further investigation.

Abstract

The chapter focuses on premorbid intellectual quotient (IQ) in cannabis users, one of the most controversial topics in studies on the harmful effects of cannabis use on cognition. We critically reviewed the literature in order to propose a model that would be able to summarize the complex relationship between premorbid IQ and cannabis use, in determining different outcomes. Longitudinal studies that assessed IQ measures before and after cannabis consumption had yielded discordant findings. We, however, observed a relationship between higher premorbid IQ and recreational or discontinued use, while a lower premorbid IQ resulted as a predictor for regular or heavy cannabis use. Using these observations as a starting point, we sustain the hypothesis of a shared genetic vulnerability to psychosis and cannabis use, reflected on premorbid IQ, and able to influence the contact with the substance. In turn, cannabis may be a trigger for psychosis toward this continuum of neurodevelopmental vulnerability.

Measures of Intellectual Ability

Currently, the IQ score is based on the standard statistical distribution of scores around a mean of 100 as described earlier in the section on types of scores. One implication of clinical importance is that at the statistical scoring extremes of a population, a few points of change can greatly affect school placement decisions, whereas a few points of change around the population average can be dismissed as random error. A new treatment that shifts the mean of the affected population from 94 to 100 substantially reduces the proportion of affected children with cognitive impairment.

One advance in the development of intelligence scales has been a greater attempt to organize test items into groupings that can remain consistent throughout the age span. This improvement, developed by Wechsler, may be responsible for the continued dominance of his series of tests including the Wechsler Adult Intelligence Scale (WAIS), which preceded the development of the comparable scales for children, now the Wechsler Intelligence Scale for Children–IV (WISC-IV) and the Wechsler Preschool and Primary Scale of Intelligence–Revised (WPPSI-R). Another important trend in intelligence test construction is the notion of subtypes of intelligence. In contrast to the original Stanford Binet, which yielded one global IQ score, measures that divide performance into at least two domains of verbal and nonverbal or performance abilities are now favored. These subtests of intelligence tests provide insight into the strengths and weaknesses of a child.

Sometimes intelligence tests are supplemented by additional measures of special ability, such as Visual Motor Integration, Wide Range Assessment of Memory and Learning, measures of information processing such as the Detroit Tests of Learning Aptitude–3, or language tests, to describe a child's skills or deficits further. A detailed review of various cognitive tests and approaches to administration and interpretation may be found in Sattler's authoritative text (Sattler, 2001) (see also Chapter 81).

Some limitations of attempts to divide intelligence into multiple domains may be related to measurement limitations, rather than the actual nature of individual abilities. When subscales contain few items, they are less likely to be reliable. They also are more likely to be in error. Some purported subtests do not prove to be statistically distinct. Using data from a variety of sources, however, Gardner (1983) has shown evidence for distinct “multiple intelligences.” The different abilities include logical-mathematical, spatial, bodily kinesthetic, musical, interpersonal, intrapersonal, and naturalist. Intelligence tests provide only a limited view of the abilities of individuals.

IQ test scores may be invalid if the child has a sensory or motor impairment that limits his or her ability to understand instructions and other such factors can adversely affect IQ test results. The diagnosis of intellectual disability or cognitive impairment requires not only low IQ test scores, but also comparable scores on a test of adaptive behavior (see Chapter 68). Some tests of adaptive behavior, such as the Vineland, generate standard scores, facilitating comparisons with IQ test scores.

Clinical features

Most patients have an IQ between 50 and 75, and often live at home. However, patients with severe learning impairment (IQ below 50) are often totally dependent on others. Brain damage may cause not only mental but also physical impairment and epilepsy; visual defects, hearing, speech or behavioural disorders, facial deformities or cardiac defects are often associated. The limitations cause children to learn and develop more slowly than normal children. They typically take longer to learn to speak, walk and take care of their personal needs, such as dressing or eating, and often have trouble learning. They may sit up, crawl, walk or talk late, or have difficulty speaking. They may find it hard to remember things, and have trouble solving problems or thinking logically. They may also find it difficult to understand social rules and how to pay for things and can have trouble seeing the consequences of their actions.

Other problems may include psychiatric disorders (symptoms are often modified by poor language development and other defects), hyperkinesis and stereotyped movements (body-rocking and self-mutilation are common; Fig. 28.7). Feeding difficulties may be present. Pica (the ingestion of inedible substances) is also fairly common.

6.1 Summary of the Results for the Client (Bryan, Teacher, School, and Parents)

Bryan has an average IQ, his perceptual organization is more than 1.5 SD below average, his attention and concentration are more than 1 SD below average, his relationship with the teacher is very tense, and Bryan avoids going to school. His self-esteem and achievement motivation are average.

Advice for treatment was concrete, and its implementation was feasible. It was considered that it would be better if Bryan could change class for the next year. The relationship with the teacher had to be improved. A daily planning of tasks was made and agreed on by both Bryan and the teacher, and the tasks were divided into small units. Instructions were short and clear, a time limit was given for each task, and he could ask questions at fixed times. The teacher was informed about the assessment results.