We know that there is a genetic basis for schizophrenia, which is a disorder of brain development. And so, if you have a family history of schizophrenia you're probably more likely to have it. But, the risk is still small. If you have an identical twin who has the same genes as you and they have schizophrenia, your risk of developing schizophrenia is still only 50 percent, which is high, but it's not 100 percent.
If you have a parent or a sibling who has schizophrenia, your risk is more like 10 percent. The population general risk is one percent, and so it is an elevated risk. But, just because you have a family member who has it doesn't mean that you will have it. So, there's the genetics, and we don't know which genes are involved in schizophrenia. It's probably multiple genes. We do have a few candidate genes that we're focusing on in the research.
So, there are genes, but then there's the environment, too. And what we know is that the prenatal and perinatal period, the period of pregnancy, birth and delivery, and just after delivery, is a sensitive period for brain development. So, there has been associations between maternal malnutrition, exposure to toxins and viruses, and maternal bleeding during pregnancy, as well as low birth weight. Those things have been associated with development of schizophrenia later on in life.
Again, just because those things have occurred does not mean that schizophrenia will occur. It's a very small minority of people who experience those risk factors that will develop schizophrenia, but it does increase risk. Factors during childhood, adolescence and early adulthood that may be implicated include traumatic or very stressful life circumstances, so child abuse, and then hardships throughout early life contribute to the development of schizophrenia.
Substance use, and in particular marijuana use during adolescence and early adulthood, have been linked to the earlier onset of and risk for schizophrenia. All around the world, the risk for schizophrenia is about one percent of the population that has it. From country to country, it's the same, so that we know that race and nationality and culture are not independent risk factors for schizophrenia. However, immigration and the stress of immigration may lead to increased risk of developing schizophrenia.