In studies where people are questioned about something that occurred in the past, their recollections may be affected by current realities. Recall bias, as this phenomenon is known, can be a major problem when researchers are investigating what factors could have led to a health condition, and interviews are the prime source of information. For example, since there's a widespread — though unsubstantiated — belief that autism is somehow caused by the measles-mumps-rubella (MMR) vaccine, parents of children on the autism spectrum are more likely to recall that their child was immunized prior to showing signs of autism, and draw a connection between the two events [source: Pannucci and Wilkins].
Similarly, mothers of children with birth defects may be more likely to remember drugs that they took during pregnancy than mothers of fully abled children. One study also found that pilots who knew they had been exposed to the herbicide Agent Orange had a greater tendency to remember skin rashes that they experienced in the year after exposure [source: Boston College].