A study of adults who suffer from migraine headaches revealed that a significant proportion of the study participants suffer from a complex syndrome characterized by a set of three symptoms  Those who suffer from the syndrome experienced excessive anxiety during early childhood  As adolescents, these people began experiencing migraine headaches  As these people approached the age of 20, they also began to experience recurring bouts of depression
Since this pattern is invariant, always with excessive anxiety at its beginning, it follows that excessive anxiety in childhood is one of the causes of migraine headaches and depression in later life
It does not specify the proportion of those in the general population who suffer from the syndrome.