Etiology and Conditions
Factors Causing or Contributing to Intellectual and Developmental Disabilities
Maire Percy, Ivan Brown, and Wai Lun Alan Fung
WHAT YOU WILL LEARN
•Importance of education about factors that cause or contribute to intellectual and developmental disabilities (i.e., risk factors)
•Overview of causal or contributing factors
•Biomedical, social, behavioral, and educational risk factors
•Prevention of intellectual and developmental disabilities
•Comorbid sensory impairments and mental health disorders
•Future directions in the field
The purpose of this chapter is to draw awareness to different factors that can cause or contribute to intellectual and/or developmental disabilities. There are several reasons why this chapter is important. First, obtaining an explanation for why a person has been diagnosed with intellectual disability as early as possible (i.e., an early diagnosis) provides relief to families and others who care for the child. Second, an early diagnosis facilitates access to supports and services earlier than in the absence of a diagnosis; in some cases, these may prevent actual or further impairment from developing. Third, an early diagnosis may help to prevent recurrences of certain types of intellectual and developmental disabilities within an affected individual’s family. Fourth, having information about the causes of intellectual and developmental disabilities helps administrators and policy makers allocate funding for supports and services, because these often are geared to specific types of disabilities. Finally, before an explanation is sought for what may have caused intellectual or developmental impairments, professionals and families should consider any negative consequences that might arise from a child becoming “labeled.” Sometimes, complex ethical, legal, and social issues can arise that might interfere with obtaining medical or life insurance or with employment, or labels might upset family members. Should such concerns arise, it is important that appropriate guidelines, laws, policies, or strategies be developed and implemented to guard against them (Percy, 2007; Percy & Brown, 2011).
Using criteria in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR), intellectual disability has been estimated to affect approximately 2%–3% of the general population worldwide, the prevalence being lower in developed than developing countries (American Psychiatric Association [APA], 2000). On the basis of DSM-5 criteria, the prevalence is approximately 1% (APA, 2013). However, in a study of 1997–2008 data representative of U.S. households, as many as 1 in 6 children in the United States is affected by some type of developmental disorder, including attention-deficit/hyperactivity disorder (ADHD), intellectual disability, cerebral palsy, autism spectrum disorder (ASD), seizures, stuttering or stammering, moderate to profound hearing loss, blindness, learning disorders, and/or other developmental delays (Boyle et al., 2011). Most cases of intellectual disability are mild, with less than 0.5% being severe (Rauch et al., 2012).
Intellectual disability is sometimes classified as syndromic (in which intellectual impairments associated with other medical and behavioral signs and symptoms are present) and nonsyndromic (in which intellectual impairments appear without other medical and behavioral signs and symptoms). Syndromic intellectual impairments account for 30%–50% of cases (Kaufman, Ayub, & Vincent, 2010; Rauch et al., 2012; Srour & Shevell, 2014.)
Many different factors that cause or contribute to intellectual and developmental disabilities have been identified. One quarter to one half of intellectual or developmental disability diagnoses are associated with genetic factors (Srour & Shevell, 2014). Although some publications report that the cause of intellectual disability is unknown in up to half of cases, a population study in a middle-income country identified causal factors in approximately 90% of a birth cohort (Karam et al., 2015). Studies of children indicate that 1.5 times as many males are affected with intellectual disability as females, but the male-to-female proportion decreases with increasing severity of intellectual impairment (McLaren & Bryson, 1987). In autism spectrum disorder (ASD), the most widely reported male-to-female ratio is 4-5:1 (Lai, Lombardo, Auyeung, Chakrabarti, & Baron-Cohen, 2015).
The American Association on Intellectual and Developmental Disabilities (AAIDD) focuses on four different types of factors that cause or contribute to intellectual disability (biomedical, social, behavioral, and educational) and on the timing of exposure to these factors (prenatal, perinatal, and postnatal) (AAIDD, 2010). Biomedical factors include genetic disorders and various factors adversely affecting health. Social factors include adverse family social interactions, lack of access to health care, and parental neglect. Behavioral factors include any behavior that adversely affects functioning, such as maternal alcohol or substance abuse. Educational factors include lack of accessibility to educational experiences that support adaptive skills, such as family support and/or special education, especially early in life. Sometimes the term environmental is used to denote health-related, social, behavioral, and educational factors. Prenatal means occurring before birth; perinatal relates to the period shortly before and after birth (traditionally, from the 20th week of gestation to the 28th day of newborn life). Postnatal means after birth (traditionally, the first 6 weeks after birth). Nongenetic factors are sometimes referred to as environmental factors.
Factors involved in intellectual and developmental disabilities are often referred to as risk factors. Many forms of intellectual and developmental impairment are thought to result from more than one factor (McLaren & Bryson, 1987). Risk factors can be causal or contributing. The term causal implies that the factor (or factors acting in combination) actually causes the intellectual impairment or developmental delay (i.e., that the probability of causing the intellectual impairment or developmental delay is 100% or close to this). Contributing implies that the factor(s) in question is/are not sufficient on its/their own to cause the intellectual impairment or developmental delay. Conversely, some factors can help to prevent intellectual impairment or developmental delay or reduce its severity. The study of factors involved in intellectual and developmental disabilities is complex and challenging. For example, having an extra chromosome 21 is known to cause Down syndrome, and Down syndrome is associated with intellectual disability. However, the degree of intellectual disability in people with Down syndrome is highly variable, and the nature of this variability is not yet understood. This may involve multiple biomedical and/or psychosocial factors. Susser (2002) noted that ideas of what are causal factors in human disease have “changed over the years as societies, understanding of disease, and technical resources have changed.” Furthermore, this field of study continues to evolve (Xiang et al., 2015).
An introduction to risk factors for intellectual and developmental disabilities should include a brief review of genetics, the “nature versus nurture” debate, the subspecialty of genetics called epigenetics, and brain plasticity.
A Brief Review of Genetics
Every cell in the human body contains 46 chromosomes: 23 from the mother and 23 from the father. Chromosomes are structures in the nucleus of the cells made up of tightly coiled strands of DNA. Along these strands are sections referred to as genes. Genes contain information that both enables the body to grow and work and determines how the body grows and works. Genes are passed from parents to children. Twenty-two of the 23 chromosome pairs are called autosomes; these are not involved in sex determination. The chromosomes that determine sex (the sex chromosomes) are the X in females and the Y in males. Females have two X chromosomes; males have one X and one Y. DNA is also present in mitochondria, the energy-producing organelles of cells. Mitochondrial DNA is inherited from the mother. When the first cell is formed from the mother and father, mitochondria from the father are destroyed. In this first cell, very occasionally a chromosome (or more) is structurally not normal, or there can be too many or too few of them, or they can be changed in very minor ways. Sometimes genetic abnormalities are inherited from one parent or from both parents, but sometimes they are not and occur de novo (i.e., spontaneously; Nussbaum, McInnes, & Willard, 2015). Inh...