A common problem with some scientific research,particularly projects studying human health, is that it is often difficult, and if not impossible,to prove that a specific action directly causes an effect. For example, scientists maintain found that those who smoke cigarettes also are more likely to suffer from depression. However, or scientists cannot uniquely determine whether smoking directly causes depressive symptoms,or if those with depression are more likely to cause health damaging behaviors, including smoking.
Source: phys.org