Another robust epidemiological feature is that, compared with rural regions, being born and/or raised in urban areas is associated with an increased risk of developing schizophrenia.
The quality of evidence for place of birth as a risk factor has been strengthened by Dutch and Danish population-based studies. They found the relative risk of developing schizophrenia when born in the city versus being born in the country was about 2.4 and that there was a dose–response relationship (the larger the town of birth, the greater the risk) suggesting a causal effect. Because urban-birth is relatively frequent, they reported the population attributable fraction (PAF) for this variable to be substantial (about 30%). In other words, because so many people are born and live in cities, a relatively small increase in risk could cause a large increase in those developing schizophrenia. This conclusion is supported by our recent systematic review which found that the incidence of schizophrenia is higher in urban areas. Urban birth may have a greater overall impact than heredity factors; eg one investigator compared the (PAF) for urban birth (over 30%) with 9% or 7% for having a mother or father with schizophrenia, respectively.
Place of birth could be a proxy marker for a risk-modifying variable operating at or before birth (such as such as lower levels of sunlight or greater exposure to infection; discussed later). However, since most people who are born in a city are also brought up there, it is difficult to disentangle pre- and perinatal effects from those operating later in childhood. Indeed, there is some recent evidence that the more years spent in an urban area during childhood, the greater the risk of developing schizophrenia.