A definition of Mental Retardation and the link between lead poisoning and mental illness

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Mental Retardation & the link to Lead Poisoning

This entry considers the definition of ‘mental retardation’ and the link between lead poisoning and mental illness.

The term ‘mental retardation’ may seem outdated to us because of the pejorative use of the words ‘mental’ and ‘retarded’ but the clinical application of the term is quite specific.

In the case of the child who is central to Object Retrieval, the term ‘mental retardation’ is made more unsettling by the implied racial connection to ‘Non-European’ extraction. Although it is not overtly stated, there seems to be an underlying causal link being hinted at by the person writing up the case notes.

The following extended extract taken from The Developing Child by Helen Bee outlines the definition of ‘mental retardation’ from a clinical and social perspective.

The Developing Child (eighth edition)
by Helen Bee
Longman (An imprint of Addison Wesley Longman, Inc.)



Mental retardation is normally diagnosed when a child has an IQ below 70 and has significant problems in adaptive behaviour – such as an inability to dress or eat alone or a problem getting along with others or adjusting to the demands of a regular school classroom. Thus, a low IQ score is a necessary but not sufficient condition for an individual to be classed as retarded. As Thomas Achenbach (1982) says, “Children doing well in school are unlikely to be considered retarded not matter what their IQ scores” (p.214).

Low IQ scores are customarily divided up into several ranges, with different labels attached to each, as you can see in Table 15.3 I’ve given both the labels used by psychologists, and those that may be more common in the school system. (There are no school system labels for children with IQs below 35 because schools very rarely deal with children functioning at this level.)

Table 15.3
IQ Scores and Labels for Children Classed as Retarded

Approx. IQ Label Used by Psychologists Label Used in Schools
68-83 Boderline retarded (No special label)
52-67 Mildly retarded Educable mentally retarded (EMR)
36-51 Moderately retarded Trainable mentally retarded
19-35 Severely retarded (No special label)
Below 19 Profoundly retarded (No special label)

The farther down the IQ scale you go, the fewer children there are. More than 80 percent of all children with IQs below 70 are in the “mild” range; only 2 percent of low-IQ-youngsters (perhaps 3500 children in the U.S.) are profoundly retarded (Broman et. al., 1987).

Cognitive Functioning in Retarded Children
In Chapter 7 I mentioned that some researchers interested in information processing have tried to understand normal intellectual processing have tried to understand normal intellectual processing by looking at ways in which retarded children think or approach problems differently that do normal-IQ children (Campione, Brown, & Ferrara, 1982; De-Loache & Brown, 1987). The major conclusions from this research are that retarded children:

1. Think and react more slowly.
2. Require much more complete and repeated instruction to learn new information or a new strategy (compared to normal-IQ children, who may discover a strategy for themselves, or profit from incomplete instruction).
3. Do not generalize or transfer something they have learned in one situation to a new problem or task. They thus appear to lack those “executive” functions that enable older, higher-IQ children (or adults) to compare a new problem to familiar ones or to scan through a repertoire of strategies until they find one that works.

On simple tasks, retarded children learn in ways and at rates that are similar to younger normal-IQ children. The more significant deficit lies in higher-order processing. These children can learn, but they do so more slowly and require far more exhaustive and task-specific instruction.

Causes of Retardation
About 15 to 25 percent of mentally retarded children have an identifiable physical problem (Broman et al., 1987). Chromosomal anomalies such as Down syndrome or the Fragile X syndrome are a major culprit. As Scarr and Kidd put it, “Having too much or too little [genetic material] will affect intelligence, always for the worse” (1983, p.380). A child may also inherit a specific disease or inborn error of metabolism that can cause retardation if not treated. The best-known inherited metabolism error is phenylketonuria (PKU), which I described in Chapter 2 (Table 2.2).

A third physical cause of retardation is brain damage, which can result from a large number of causes, including prenatal maternal diseases like syphilis or cytomegalovirus, prenatal malnutrition, or alcoholism. Brain damage may also occur during birth or be caused by an accident later (e.g., an auto accident or falling out of a tree house on your head).

The remaining three-quarters of retarded children show no signs of brain damage or other physical disorder. In almost all such cases, these children come from families in which the parents have low IQs, family life is highly disorganized, the parents are mentally ill, or emotional or cognitive deprivation exists. Often both genetic and environmental influences operate simultaneously.

Large-scale studies have now shown quite conclusively that these several causes of retardation are not distributed evenly across the range of low IQ scores. The lower the IQ, the more likely it is that the cause is physical rather than environmental.


With regard to mental illness and lead poisoning, it is interesting to note the research made by Nathan W. Bower et al., ‘Human lead exposure in late 19th century mental asylum population’. [Available at ScienceDirect.com / Science of the Total Environment 372 (2007) 463-473] It establishes a clear link between lead exposure and mental illness. The abstract is outlined below.


Lead isotope ratios and lead (Pb) levels were analyzed in 33 individuals from a forgotten cemetery at the Colorado Mental Health Institute at Pueblo, Colorado dating to 1879-1899. Isotopic ratios from healing bone fractures, cortical bone, and tooth dentine provide information about sources of Pb exposures over a range of time that illuminates individual’s life histories and migration patterns. Historical records and Pb production data from the 19th century were used to create a database for interpreting Pb exposures for these African, Hispantic and European Americans. The analysis of these individuals suggests that Pb exposure noticeably impacted the mental health of 5-10% of the asylum patients in this frontier population, a high number by standards today, and that differences exist in the three ancestral groups’ exposure histories.

The Pb production statistics summarized here provide a context so that sources of Pb exposure for individual 19th century North Americans can be discerned from their skeletal Pb isotopic ratios. Lead isotope rations for the Colorado Mental Health Institute skeletal population correlates well with the distribution fo sources expected for a Colorado immigrant population from the late 19th century, a period for which relatively few analyses of human Pb levels or isotopic rations have been published. Lead levels found in bone and teeth suggest that differences in environmental exposure existed between different ancestral groups with the highest levels in European Americans and the lowest in Hispanic Americans. Lead exposure, primarily from smelters, created a heavy burden for community residents that increased their chance of suffering mental difficulties by 5% to 10%.


Even more compelling for the project Object Retrieval is the link made by Sir Alan Moncrieff, who was the clinician who dealt with the boy concerned. It makes a link to mental retardation and levels of lead in the blood.


[…] In 1964, Sir Alan Moncrieff and others at the Institute of Child Health in London found that a group of mentally retarded children had distinctly more lead in their blood than a group of normal children. In fact, nearly half the retarded children had higher blood levels than the maximum level in the other group. It does not, of course, follow that lead was responsible for the children's mental retardation. It could well have been their retardation which made them more prone to chew on substances with a lead content. Nevertheless, the possibility that lead at levels too low to cause obvious poisoning could result in mental retardation could not be ignored and acted as a spur to the search for some measurable effect of low levels of lead in the human body. […]