How Common Is Autism Spectrum Disorder?

The rate of autism spectrum disorder appears to have risen noticeably over the past thirty years. Most of this rise is explained by broader diagnostic criteria, greater awareness, and the wider use of screening; but whether there has been some increase in the actual rate is still a matter of debate among scientists. In this chapter we'll go into detail about the current data, how it's gathered, and why it varies.

Current Prevalence Data

According to the most recent data from the Autism and Developmental Disabilities Monitoring Network (ADDM) of the U.S. Centers for Disease Control and Prevention (CDC), about 3.2 percent of 8-year-old children, that is, about 1 in 31, are identified with autism. This rate has risen from about 1 in 36 in the assessment two years earlier; when the first ADDM report was published in 2000, the rate was around 1 in 150. The World Health Organization estimates the average global rate at about 1 in 100 children; the differences between countries largely reflect differences in access to diagnostic services.

While comprehensive nationwide screening studies in Turkey are still limited, clinical observations and regional studies suggest that the rate is in a range similar to international data. In recent years, the spread of developmental screening in our country (especially at the 18- and 24-month checkups) shows that diagnosis rates have risen in parallel with international trends.

How Should We Interpret the Rise in the Rate?

In interpreting this noticeable rise in the rate, four main factors need to be kept in mind:

• Broader diagnostic criteria — including presentations previously considered separate, such as Asperger's syndrome and PDD-NOS, under the autism umbrella with DSM-5 has increased the number of children diagnosed.
• Greater awareness — parents, teachers, and primary care doctors are able to notice the signs of autism and make referrals much earlier than before.
• Wider screening programs — including routine developmental screening (see Chapter 16) in child health monitoring has allowed children with milder presentations who previously went unnoticed to be diagnosed as well.
• A possible real increase in the rate — even after adjusting for the three factors above, whether there is a small real increase is a matter of debate among scientists; there is as yet no firm evidence of one.

The Boy-to-Girl Ratio

Autism is identified noticeably more often in boys than in girls; current data show the boy-to-girl ratio to be between about 3.4-to-1 and 4-to-1 (about 1 in 20 boys and about 1 in 70 girls). But whether this ratio reflects a real biological difference in rate, or the fact that diagnosis is delayed and missed in girls, is an important matter of debate. The different presentation of autism in girls will be covered in detail in Chapter 11 of the book.

Socioeconomic and Regional Differences

In the past, a diagnosis of autism was recorded more often in children of families with higher socioeconomic status; this came not from a real difference in rate, but from those families being able to reach assessment services more easily. In recent years, as screening and access have become more widespread, this gap has narrowed and in some regions has even reversed. This change strongly supports the idea that a large part of the differences in rate has to do with access to diagnosis.

Info Box

Most of the rise in the rate is explained by better recognition and broader diagnostic criteria.

Autism occurs in all racial, ethnic, and socioeconomic groups; it isn't specific to any particular community or culture.

The rise in early diagnosis rates is actually a positive development: more children are able to reach the support they need earlier.

The Change in Age at Diagnosis

While in the past a diagnosis of autism was usually made toward the end of the preschool years, today, with greater awareness and the spread of screening tools (see Chapter 16), the age at diagnosis has dropped noticeably. In some regions the median age at diagnosis has fallen below 3 years; at experienced centers, a reliable diagnosis can be made as early as 18–24 months. This early diagnosis is extremely valuable in making better use of the window of opportunity for early intervention; but it should honestly be noted that many families still go through a waiting process that lasts months, even years, from the first concern to diagnosis.

Autism in Adulthood

Autism isn't specific to childhood; it continues throughout life. According to CDC estimates, about 2.2 percent of adults are autistic. Today there are many people who receive a diagnosis for the first time in adulthood, having gone unnoticed in childhood; this is seen more often in girls and women and in people with a milder presentation. The topic of being diagnosed in adulthood will be covered in Chapter 45 of the book.

Chapter Summary

In this chapter we looked at the current rate of autism, how that rate has changed over time, and the reasons behind that change. In the next chapter, we'll take a deep look at what science answers to the question "Why my child?", the genetic and environmental causes.