a large proportion of autism is caused by CNVs and is detectable via array CGH, or that detecting CNVs is tantamount to a genetic diagnosis. The Autism Genome Project database contains genetic linkage and CNV data that connect autism to genetic loci and suggest that every human chromosome may be involved. It may be that using autism-related subphenotypes instead of the diagnosis of autism per se may be more useful in identifying susceptible loci. Twin studies Twin studies are a helpful tool in determining the heritability of disorders and human traits in general. They involve determining concordance of characteristics between identical (monozygotic or MZ) twins and between fraternal (dizygotic or DZ) twins. Possible problems of twin studies are: (1) errors in diagnosis of monozygocity, and (2) the assumption that social environment sharing by DZ twins is equivalent to that of MZ twins. A condition that is environmentally caused without genetic involvement would yield a concordance for MZ twins equal to the concordance found for DZ twins. In contrast, a condition that is completely genetic in origin would theoretically yield a concordance of 100% for MZ pairs and usually much less for DZ pairs depending on factors such as the number of genes involved and assortative mating. An example of a condition that appears to have very little if any genetic influence is irritable bowel syndrome (IBS), with a concordance of 28% vs. 27% for MZ and DZ pairs respectively. An example of a human characteristics that is extremely heritable is eye color, with a concordance of 98% for MZ pairs and 7–49% for DZ pairs depending on age. Identical twin studies put autism's heritability in a range between 36% and 95.7%, with concordance for a broader phenotype usually found at the higher end of the range. Autism concordance in siblings and fraternal twins is anywhere between 0 and 23.5%. This is more likely 2–4% for classic autism and 10–20% for a broader spectrum. Assuming a general-population prevalence of 0.1%, the risk of classic autism in siblings is 20- to 40-fold that of the general population. Notable twin studies have attempted to shed light on the heritability of autism. A small scale study in 1977 was the first of its kind to look into the heritability of autism. It involved 10 DZ twins and 11 MZ twins in which at least one twin in each pair showed infantile autism. It found