If we condition on a descendant of 𝑇 that isn’t a mediator, it could unblock a path from 𝑇 to 𝑌 that was blocked by a collider. For example, this is the case with conditioning on 𝑍 in Figure 4.13. This induces non-causal association between 𝑇 and 𝑌 , which biases the estimate of the causal effect. Consider the following general kind of path, where → · · · → denotes a directed path: 𝑇 → · · · → 𝑍 ← · · · ← 𝑌 . Conditioning on 𝑍 , or any descendant of 𝑍 in a path like this, will induce collider bias. That is, the causal effect estimate will be biased by the non-causal association that we induce when we condition on 𝑍 or any of its descendants (see Section 3.6).