This is the first part of a combined theory of autism and general intelligence (IQ). It is argued that general impairment of gene-expression, produced by a diversity of environmental and genetic causes, is in moderation advantageous in suppressing genetic idiosyncracies. But in excess it will produce a condition involving abnormalities of appearance and behaviour, with a particular relationship to high parental social class and IQ and with particular sex distributions. Character-istics and findings relating to schizophrenia, manic-depressive illness, or neuroses indicate that they cannot reasonably be considered manifestations of excessive general impairment of gene-expression. By contrast, characteristics and findings relating to autism accord very well with this conception. The suggestion is that autism involves primary abnormalities in diverse parts of the brain and in diverse psychological functions. Random binding to DNA may be a substantial mechanism of general impairment of gene-expression. [i.e., would definitely cause impairment, and hence cause autism, but only may be substantially involved (see para. 15)].