Lurking variables are a challenge to good experimental designs. Lurking variables are variables that aren't being studied or controlled for but that have an effect on experimental results. In medical experiments, real effects often come from no physical cause, and those effects must be taken into account when testing new medications (thus we need to consider the placebo effect). For example, if we give some people a new drug for curing the common cold, and they get over their colds more quickly than people who did not take the drug, was it the drug or the placebo effect that shortened the cold?
To help identify these lurking variables, let's discuss the following about experiments related to child-rearing techniques and the nature vs. nuture debate. Why are behavioral studies like this so difficult to conduct?