How is causality usually proven when it's impossible to change the independent variable? Also, why is correlation often treated as a very reliable hint to causation, even in various respected journals? Here's an example: http://www.webmd.com/diet/news/20080303/eating-breakfast-may-beat-teen-obesity This article directly implies a causality between eating breakfast and weighting less ("Although adolescents may think that skipping breakfast seems like a good way to save on calories, findings suggest the opposite."), based only on a correlation. However, there can be many causes for the said correlation. For example, those who are more physically active (which is a cause for a weight loss) may be more likely to eat breakfast. Or those who sleep less may be more likely to skip breakfasts, and also have a slower metabolism as a result of sleep deprivation. Doesn't one need to conduct an experiment and see if manipulating the breakfast independent variable will actually lead to a reduction in weight?