In my studies I found this very interesting. The MacDonald triad consists of three traits many serial killers have in common with each other, even if their methods vary.
1. Enuresis- Uncontrollable bedwetting.
This can be as a child, teenager, or as an adult. It's just one of those traits that have been documented as a marker for serial killers. Often times in the hiring process for law enforcement, or military, the psychology evaluations will ask you questions like "have you ever wet the bed?" several times over in different forms. This is why.
2. Fire starting-
A general fascination with fire and the effects of fire can be a major trait for a serial killer. the three major "serial" offenses are murder, arson, and sexual assault.
3. Cruelty to animals-
The train of thought on this trait is that a young would-be serial killer practices his/her skills in death on small animals, usually household pets or other people's pets, to in a sense "alleviate" the itch of killing at an early stage in life.
These are the three most looked at. Just thought I would share this- It's interesting how often these come up in criminal profiling.
If you are interested in more criminal profiling methods and how to profile people yourself, the link below should instant download a free e-book on Criminal Profiling and how to do it, directly from my dropbox. It's a PDF, and after the boring introduction, teaches you many interesting things. Enjoy!