Two studies, one in the United States (done by David Greenberg, a geneticist at Columbia University in New York) and other in Britain (done by Christopher Gillberg of St George's Hospital Medical School in London), found a higher rate of autism among twins. The rate of autism in identical twins was 12 – 14 times that of the general population, and the rate of autism in fraternal twins was about 4 times higher than the general population. Another study presented at a conference in Boston in 2005 postulated that birth month greatly affects the chances of twins suffering from autism with twins born in January having an 80% higher chance of developing the disorder than twins born in December.

While this information may seem scary at first, it's important to note two things: most of these studies relied on small samples groups, which may not provide the most reliable information. Also, the Columbia study limited itself to twins with siblings with autism, which has been identified as a risk factor for developing the disorder, and may have skewed the results. Finally, most twins will not suffer from autistic disorders, and even if one twin does develop autism, there is no guarantee that the other will develop it as well (even amongst identical twins). One mother of twins kept a daily diary on her fraternal twins' development; through this diary, one can see signs of autism in one of the twins dating from six months of age. Now twelve years old, one twin is autistic, and the other is unaffected. However, the mother's documentation of the twins development opened the eyes of researchers and parents as to the pervasiveness of this disorder, and the possibility that it can be identified much earlier than previously thought.

For years, scientists, parents, and doctors have debated the causes of autism. At first, twin studies seemed to point to a genetic cause for the disease; however, the increased risk amongst fraternal twins (though not as high as that of identicals) seems to indicate that perhaps something about being a twin could also be a factor. This means that environmental factors in the womb, placental development, or even the experience of being raised with a same-age sibling could have some triggering effect for autism.

Some authorities estimate that the incidence of autistic disorders has increased over 172% since 1990. Others argue that the incidence of autism has remained static, but our ability to identify and assist autistic children has improved. For more information on the studies cited in this article, and to access information on having twins assessed for autism, see the links below:
Excess of Twins among Affected Sibling Pairs with Autism: Implications for the Etiology of Autism David A. Greenberg, Susan E. Hodge, Janice Sowinski, and Doug Nicoll from The American Journal of Human Genetics
https://www.journals.uchicago.edu/AJHG/journal/issues/v69n5/013031/013031.html

Increased Rate of Twins among Affected Sibling Pairs with Autism
Catalina Betancur, Marion Leboyer, and Christopher Gillberg
from The American Journal of Human Genetics
https://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=447617

AWARES article on Boston Autism Conference
https://www.awares.org/pkgs/news/news.asp?showItemID=546&board=&bbcode=&profileCode=§ion=

Smile for the Camera—and for science (article about mom who kept twin diary) from the age.com.au
https://www.theage.com.au/articles/2005/03/19/1111086056480.html

Warning Signs and Symptoms of Autism and Pervasive Developmental Disorder (PDD) from Autismweb https://www.autismweb.com/signs.htm