There have been several categorizations of pedophilia, but perhaps the most useful involves the distinction between true pedophiles and opportunistic pedophiles. Similar terms include fixated vs. regressed pedophiles and preferential vs. situational pedophiles. True pedophiles have persistent sexual attraction to pre-pubescent children. Opportunistic pedophiles have less of a focused sexual attraction to children. Their sexual engagement with children may depend on circumstances, such as the convenient availability of a child victim, loss of normal inhibitions due to substance abuse, or difficulty feeling comfortable with an adult sexual partner. In this way, opportunistic pedophiles may overlap with non-pedophilic child molesters.