A robust epidemiological feature of schizophrenia is the slight excess of births in winter and spring in the temperate regions of the Northern Hemisphere. This points towards a risk factor (such as lower winter sunlight or increased winter infection) operating pre- or peri-natally rather than around the time of onset,
Also, while the seasonal excess is small it may have a larger impact. For example, a comprehensive Danish record-linkage study found that the small seasonal excess of schizophrenia births (relative risk = 1.11) was associated with a sizeable (10.5%) population attributable fraction (PAF) for the disorder. The reason for this high PAF is that birth in winter or spring is a common exposure.
In other words, because so many people are born winter or spring, a relatively small increase in risk would cause a large increase in the numbers of people with the schizophrenia.