Elsevier

Psychiatry Research

Volume 166, Issue 1, 31 March 2009, Pages 85-90
Psychiatry Research

Why did the savant syndrome not spread in the population? A psychiatric example of a developmental constraint

https://doi.org/10.1016/j.psychres.2007.11.010Get rights and content

Abstract

A developmental constraint is a mechanism that limits the possibility of a phenotype to evolve. There is growing evidence for the existence of developmental constraints in the biological literature. We hypothesize that a developmental constraint prevents the savant syndrome, despite its positive aspects, from spreading in the population. Here, the developmental constraint is the result of the high interactivity among body parts in an early stage in embryological development, namely early organogenesis or the phylotypic stage. The interactivity during this stage involves all components of the embryo, and as a result mutations that affect one part of the embryo also affect other parts. We hypothesize that a mutation, which gives rise to the development of the positive aspects of the savant syndrome (e.g., an impressive memory capacity), will virtually always have a deleterious effect on the development of other phenotypic traits (e.g., resulting in autism and/or impaired motor coordination). Thus, our hypothesis states that the savant syndrome cannot spread in the population because of this developmental constraint. The finding that children with savant syndrome often have autism and physical anomalies, which are known to be established during early organogenesis, supports our hypothesis.

Introduction

A developmental constraint is a developmental mechanism or process that limits the ability of a phenotype to evolve, or biases it along certain pathways (Maynard-Smith et al., 1985, Amundson, 1994, Amundson, 2005, Beldade et al., 2002, Schwenk and Wagner, 2003). In the biological literature, the evidence for the existence of developmental constraints is growing (Amundson, 2005). In the psychological literature, the concept of constraint is often considered, but is generally accorded a different meaning. In the present article, we first explain the different conceptualizations of the term constraint in the fields of psychology and biology. Then we discuss biological examples to illustrate the concept of developmental constraint, and we present what we contend to be an instance of a developmental constraint in the psychiatric context. Specifically, we hypothesize that it is a developmental constraint that prevents the positive aspects of the savant syndrome from becoming established in the population through the process of natural selection.

Section snippets

Conceptualizations of the concept of constraint

In the psychological literature, the term constraint is generally used in the context of learning. Keil (1981), in a now classic article on constraints on knowledge and cognitive development, defined constraints as ‘formal restrictions that limit the class of logically possible knowledge structures that can normally be used in a given cognitive domain’ (p. 198). Although the use of the term limit seems to imply that constraints hinder the child in its learning, it is rather believed that

Biological examples of developmental constraints

A biological example of a developmental constraint is the fact that we have five, and not more, fingers (Galis et al., 2001). This is remarkable because a common anomaly at birth is the presence of an extra finger or toe (e.g., in humans 0.1–0.2%; Castilla et al., 1996). Thus, there is variation in the number of digits, but extra digits have never become a stable pattern. An easy conclusion would be that extra digits do not have a selective advantage. However, extra digits may well be

A psychiatric example of a developmental constraint

An example of a developmental constraint in psychiatry is related to the savant syndrome. Sufferers of this rare syndrome are often referred to as idiot savants, retarded savants, or autistic savants. They combine a serious disability, often mental retardation and/or a pervasive developmental disorder, such as autism, with a spectacular ability, which stands in stark contrast to the handicap (Treffert, 2000). The ability is highly developed in a narrow range, such as music or art performance,

Discussion

We hypothesize that the failure of the savant syndrome to become established in the population through natural selection is due to a developmental constraint, as the concept is used in biology. The developmental constraint has its origins in the negative side-effects that are induced during the early organogenesis stage. These nullify the positive, potentially fitness-enhancing, aspects of the savant syndrome. As we have only considered a few lines of support, further research is clearly

Acknowledgement

We thank Conor Dolan and Willem Frankenhuis for their valuable comments on the manuscript. Annemie Ploeger was funded by the Evolution and Behaviour programme of the Netherlands Organisation for Scientific Research (NWO).

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