Current research points to brain abnormalities as the primary cause of Asperger syndrome (AS). Using advanced brain imaging techniques, scientists have revealed structural and functional differences in specific regions of the brains of normal versus AS children. These defects are believed to be caused by the abnormal migration of embryonic cells during fetal development, which affects brain structure and "wiring" and then goes on to affect the neural circuits that control thought and behavior.
For example, one study found a reduction of brain activity in the frontal lobe of AS children when they were asked to respond to tasks that required them to use their judgment. Another study found differences in activity when children were asked to respond to facial expressions. A third study investigating brain function in adults with AS revealed abnormal levels of specific proteins, correlating with obsessive and repetitive behaviors.
Scientists are also aware of a genetic component to AS and the other ASDs because of their tendency to run in families. Additional evidence for the link between inherited genetic mutations and AS was observed in the higher incidence of family members who have behavioral symptoms similar to AS, but in a more limited form. For example, they had slight difficulties with social interaction, language, or reading.
A specific gene for AS, however, has never been identified. Recent research indicates the likelihood of a common group of genes whose variations or deletions make an individual vulnerable to developing AS. This combination of genetic variations or deletions determine the severity and symptoms for each individual with AS.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.