Ideally, in order to make a cause and effect conclusion, we employ the experimental method. But sometimes we simply can't do that. No one is going to allow you to deprive their child of exposure to language in order to see if language exposure is necessary for language development - so, we must approach some questions through alternative means. To address the question of whether or not language exposure is required during a particular critical period, we could look at a case study of some tragic case where normal language exposure wasn't there - as in the case of Genie or other similar cases. Another commonly used and frequently misinterpreted form of study is the correlational study.
Correlations refer to relationships between two things - two factors or variables.
Such conclusions can't be made, although often such conclusions are made. Recently a study addressed the issue of changes in neurotransmitter levels and behavior. There has recently been some suggestion that if we were to alter serotonin levels this would fix all the world's ills. Correlational studies suggest low serotonin is bad - a very simplistic statement. Read this short article to get an understanding of the fact that behavior affects the body and the body affects behavior - beware of making cause and effect conclusions when looking at correlational data.