As I've mentioned before, research into childhood development is tricky, thanks to ethical and practical constraints on what researchers can do. Consider randomized controlled experimental studies - that is, taking a group of test subjects, dividing them at random into a group that receives some sort of experimenter-determined influence and a group that does not, and noting the differences between the two groups after. This is considered the gold standard for making causal inferences, and it is the way that, for example, we test the efficacy of drugs that are in development.
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