A study of 78 families with children diagnosed with autism found that the age of the father when the child was conceived was key to the risk of the disorders developing. Autism spectrum disorders can range from the relatively mild social engagement difficulties seen in people with Asperger's syndrome, to severe mental retardation with a profound inability to communicate. Currently in the United States an estimated 1 in 88 children have autism. The study, led by researchers in Iceland and recently published in Nature found that, as men age, the number of hereditary mutations in their sperm increased. These mutations proportionally increased the chance their offspring would develop autism or schizophrenia. It is estimated that an average of two new gene mutations occurred for every year in the father's age past puberty; meaning the chance for new mutations being passed on to offspring doubled every 16.5 years from puberty on. The Icelandic study's findings also supported three recent American studies which found that fathers were four times more likely to pass on these mutations than mothers. The average age of Icelandic fathers in the study was 33 years. But, the researchers pointed out that because there are so many factors that contribute to the health of offspring, it is not possible to say at what age this could be a concern to an expectant father.