Epidemiologic research has identified women with mild intellectual disability (IQ, 50 to 70 range) and those with severe intellectual disability (IQ, less than 50). The latter are easiest to recognize in most cultures because the severity of their disabilities necessitates special supports and services. The identification of women with mild intellectual disability is associated with life span landmarks (e.g., educational, marital, occupational, parental) because it is at these times that women with intellectual disabilities require adequate support in order to fulfill specific expectations. Identification of the woman with mild intellectual disability is therefore likely to fluctuate. For example, the highest prevalence of women with mild intellectual disability is found in the school years. After they finish school, such women may be able to hold down a job or get married and may no longer need support services. Persons with mild intellectual disabilities predominate in the total population, constituting 75% to 89% of all persons with intellectual disabilities, compared to 11% to 25% with severe intellectual disabilities.

A dramatic change in recent years for women (and men) with intellectual disabilities has been the greatly increased life expectancy, particularly for some etiologic groups. For example, for women with Down syndrome, the most common recognized cause of intellectual disability, life expectancy is now into the fourth and fifth decade, compared to age 12 in the early part of the 1940s. Curiously, females with Down syndrome may have a significant survival disadvantage compared to males with Down syndrome (8).

There are differences between the sexes in prevalence of intellectual disabilities, with greater numbers of men overall. In the severe range, the difference tends to even out; some studies show greater numbers of women. The implications of these sex differences in terms of required supports have so far not been explored.

Women with borderline intellectual disability form an invisible but very large group, poorly understood by both researchers and clinicians. Their IQs range from 70 to 85; they tend to have difficulties managing money; they have social skills deficits and difficulties solving problems; they may be illiterate. Earlier definitions of intellectual disabilities often included these persons but, with a
growing emphasis on inclusion along with attempts to reduce services, they have been forgotten (9). The current trend, if indeed their special needs are identified (10), is to label such individuals, while in the school system, as having a learning disability. Prevalence rates based on service use tend to underestimate the size of this population and may underestimate girls more than boys. This is because persons with borderline IQ (and sometimes those with mild intellectual disabilities as well) are typically not identified in school unless they exhibit behavior problems. Behavior problems are more common in boys than girls. As adults, these women may not come to the attention of services unless they have trouble caring for their children or unless they develop major psychiatric difficulties.

Autism is the most severe form of a spectrum of related disorders referred to as the pervasive developmental disorders. These disorders are defined behaviorally by impairments in socialization and communication and by a lack of behavioral flexibility (and are sometimes referred to as autism spectrum disorders). It is estimated that about 75% of persons with autism have IQ scores in the intellectual disability range and even those with higher IQs often have relatively lower levels of adaptive functioning and are referred to the intellectual disability sector for services. Evidence suggests that a substantial proportion (one quarter to one third) of persons with intellectual disabilities also have autism; the greater the cognitive impairment, the greater the prevalence (11,12). While the male:female ratio is 3-4:1 in autism without intellectual disability or with mild intellectual disability, the gender difference diminishes with increasing intellectual disability. Some evidence suggests that, when girls are affected, they are likely to be more severely disabled (13). Conversely, some girls with autism who have mild or no intellectual disability have been reported to have fewer social and communication deficits than their male peers. Some investigators have found undiagnosed autism spectrum disorders in girls diagnosed with anxiety disorders, selective mutism, and anorexia nervosa (14,15). Taken together, these findings suggest that girls with autism may have a different phenotype than boys and, in milder cases, may not be recognized as having an autism spectrum disorder (16). Persons with intellectual disabilities and comorbid autism, because of the severity of these disabilities, have historically been the focus of institutional scandal, and with the move toward deinstitutionalization, have been among the last to be reintegrated into community life (17). These persons remain a very vulnerable group requiring considerable care and supervision throughout their lives.

While a biologic etiology can be identified in 60% to 75% of persons with severe intellectual disabilities, it remains obscure for around 25% to 40% of those with mild intellectual disabilities. (In light of diagnostic advances during the past
decade, this breakdown, based on earlier epidemiologic data, may no longer be valid. Individuals are increasingly being identified with milder manifestations of established disorders that had previously been recognized only in severely affected persons. New investigative techniques, when applied to persons with mild intellectual disabilities, now detect the underlying cause of previously undiagnosable biologic entities.) However, for all persons with intellectual disabilities, less than optimal health care (e.g., undiagnosed medical disorders such as hypothyroidism or inappropriate use of medication to manage behaviors), and less than optimal support environments (e.g., inappropriate expectations or noisy and disruptive living circumstances, giving rise to stress and anxiety) can contribute to lower levels of functioning. In recent years, recognition has increased that some causes of intellectual disabilities give rise to a characteristic profile of skills, abilities, and emotional responses as well as particular vulnerabilities to other health-related problems. For example, Down syndrome (equally prevalent in males and females) is associated with increased vulnerability to certain comorbid disabilities (e.g., hearing impairment), medical (e.g., thyroid dysfunction), and mental health disorders (e.g., depressive episodes, early onset dementia). Similarly, fragile X syndrome, the most prevalent inherited cause of intellectual disability (women with the fragile X full mutation may or may not have intellectual disability), is associated with a higher than expected prevalence of certain medical disorders (e.g., seizures, mitral valve prolapse, eye problems) and mental health problems (e.g., hyperactivity, social anxiety, obsessive, compulsive, and psychotic behaviors under stress). Other genetic syndromes with documented characteristic behavioral, emotional, and/or psychiatric disturbances include Williams, 22q11.2 deletion, Angelman, Prader-Willi, Smith-Magenis (all of which are equally prevalent in women and men), Rett (almost exclusive to females), and Turner (occurs only in women) syndromes. Recognizing these “behavioral phenotypes” (and their biologic bases) is essential in designing interventions and in providing optimal supports (18).

Women (and men) with borderline intellectual disability were once referred to as having “cultural familial” retardation because of the belief that their disability was, in part, due to an impoverished environment; however, this term is less popular now. With recent genetic advances, especially newer diagnostic investigations, such as fluorescence in situ hybridization (FISH), subtelomeric screening, chromosome microdissection, and magnetic resonance spectroscopy (MRS), we now suspect that a large proportion of women in this group also have specific biologic causes to their disability (e.g., 22q11.2 deletion syndrome, fragile X, deletion 1p36.3, treatable metabolic disorders such as Smith-Lemli-Opitz syndrome, or SLOS) (19).

The cluster of abnormal patterns of behavior called autism has several different probable etiologies. Autism is associated with a higher than expected prevalence of epilepsy, neurologic symptoms, minor congenital anomalies (birth defects), maternal birth complications, and, as previously stated, intellectual disabilities. Also found in persons with autism are genetic conditions (such as fragile X), metabolic conditions (such as phenylketonuria), viral infections (such as rubella),
and congenital anomaly syndromes (such as Möbius syndrome). However, not everyone with autism has these additional diagnoses, and not everyone with these diagnoses develops autism. Autism appears to be the outcome of particular brain damage caused by a variety of biologic insults. A small but significant minority of persons with autism shows deterioration in skills and functioning during adolescence and, for some, seizures may first occur at that time.