Although there is very strong evidence for a genetic piece to schizophrenia, the lack of full concordance between identical twins shows that the environment also plays a role. We define an ‘environmental risk factor’ as any event that is not due to genes, specifically the individual differences in the DNA sequence.These events may be biological (e.g., head injuries, viral infections), psycho­logical (e.g., disrupted family relationships), or social (e.g., poverty).Over the past few decades, scientists have found evidence for environmental risk factors in at least some cases of schizophrenia. Before reviewing this research, we must make an important distinction: some environmental factors may cause or contribute to schizophrenia while others modify or change the illness in someone who is already sick. In this book we use the term ‘cause’ to refer to any factor that can produce the illness or increase the chance of illness in someone who has not yet been affected by schizophrenia. This cause does not have to be either necessary or sufficient. This means that other causes may exist that also produce the illness, and that any given cause may need to interact with other causes for the disorder to occur. We use the term ‘modifier’ to refer to anything that changes the symptoms of the illness in someone who is already affected. As we discuss in a later chapter, knowing modifiers can help with the treatment of the disorder. However, they should not be confused with causes.Scientists who study schizophrenia and other psychiatric disorders have long ago abandoned the ‘nature–nurture’ controversy. In the past, many philo­sophers and scientists had taken one of two extreme positions. Some believed that psychiatric illness was only caused by innate or genetic factors; others felt that mental illness was the sole product of adverse environmental events. Today, we know that the question ‘genes or environment?’ is too simplistic. As Dr Paul Meehl realized several decades ago, the better question is much more com­plex: ‘What group of environmental risk factors work together with which genes to produce schizophrenia?’Before discussing specific environmental risk factors that may cause schizo­phrenia we should clarify why we believe that the study of such factors is es­sential.