Some research may involve protected population groups.
This includes people who are:
minors (younger than 18)
experiencing diminished capacity
mentally or physically challenged
pregnant (particularly for those projects where physical procedures, exercises, etc., will be performed).
The IRB must ensure these research involving these population groups is ethical and legal.
Additional information is available at the US Department of Health and Human Services' Vulnerable Populations, including a "Research Involving Vulnerable Populations" video.