Mental Retardation Incidence in Yokohama City


Department of Special Education, Faculty of Education and Human Sciences, Yokohama National University

In this paper we calculate the incidence of mental retardation in the 17 year-old population of Yokohama City during 1996, based on information provided by the Yokohama City Welfare Department and Education Offices. The studied group includes 17 year-old individuals as of May 1, 1996 enrolled for third year education in the high school department of the Special School for Mental Retardation, and mentally retarded individuals born during the same period of time (year) but not in the Special School, living in Yokohama City.
The total number of individuals enrolled in the high school department of the Special School for Mental Retardation, those who underwent education in special education class in general school during the compulsory education period due to mental retardation, those enrolled in physically handicapped, hearing impaired or vision impaired schools with mental retardation, and those whose education has been postponed or have been excused due to mental retardation add up to 297. (This number that is informed by Education Office is somewhat presumptive number. But It has been confirmed that approximately 70% of these individuals have been recognized by the city's Welfare Office.) The total number of 17 year-olds in the city is approximately 40145. Based on this information (mental retardation: 297, total population: 40145) a mental retardation rate of 0.74% at 17 years of age has been calculated for Yokohama City. Considering these individuals have completed their compulsory education up to basic high school, completed by age 15, it is hard to obtain detailed information from the Education Office except for those enrolled in special education schools. Moreover, the Education Office doesn't carry out strict intellectual examinations out of respect for the children's rights. Therefore, the mentioned calculations are not completely exact.
In Japan, after completing obligatory education, most individuals with mental retardation enroll in special high schools such as mentally handicapped, hearing impaired, vision impaired, or special workshops, but others stay home, are hospitalized, or get a job. Thus, they walk very different paths. So it is difficult that we get hold of all cases of person with mental retarded.
In any way, the reason why we choose 17 year-old population in this investigation is that in this age, the specialized section of the Welfare Office carries out the process of integral recognition of the condition taking into account intellectual capacity using strict intellectual examination, due to the following two reasons:1. The condition can be recognized precisely in this age group according to medical and psychological diagnosis due to the evident (established) symptoms of mental retardation at this age. 2. The responsibility of the administration regarding children and people with mental retardation is transferred completely from the Education Office to the Welfare Office.
However, this integral recognition including strict intellectual examinations only carried out in cases where the patient or his parents request it. Besides, in cases where the patient displays multiple conditions and has requested previous recognition of another condition, mental retardation is no longer apparently recognized. Thus, the incidence obtained from the Welfare Office information is not the medical incidence of mental retardation.
As we have described, determining the exact number of mentally retarded individuals is a difficult task. This limits our study. This study was based on information provided by the Welfare Office, which performed the exact diagnosis of the condition, and by the Education Office, in an attempt to include individuals who do not need the social welfare aid due to mild mental retardation or other causes. However, we can't deny the results of our study aren't absolutely exact. The exact incidence can be greater that 0.74%.
The Japanese Health and Welfare Ministry investigation results serve as reference. The investigation was applied to randomly selected individuals, and reported a mental retardation incidence of 0.3%. However, these results are not very dependable since the study was based on self-declaration. The AAMR declare that the incidence is 2.3% to 3% according to the theoretical statistical value. The Textbook of Comprehensive Psychiatry, a representative U.S. textbook, reports under 1% incidence. According to Ireland's intellectual disability database, the incidence is 0.73%, and the simple type certificate issuance rate for mental retardation by the Welfare Office of Yokohama City is 0.57% (if employed, this certificate entitles the owner to financial assistance from territorial entities, along with tax, health and other benefits.)
Mental retardation is frequent in children and people with other conditions. Therefore, it is crucial for state and territorial entities to determine the exact incidence in order to develop welfare policies, guarantee adequate quality of life for individuals with this condition, and establish social security programs, including education and employment. Medicine benefits from this information too, enabling the development of prevention and attention programs. It is therefore expected that efforts from different sectors converge to create a study and generate an exact incidence figure.

Kanagawa Medical Psychiatrist Society
Ordinary session No. 127,Date: Saturday November 21, 1998

(originally published in Journal of disability and medico-pedagogy, Vol 5,2002)