This effect may occur when participants get more benefit from the human contact of a research study than from the actual treatment under clinical investigation.

Answer:Internal Invalidity

Explanation:

Internal Invalidity means something else produces the outcome that is observed in a research and is a result of the conclusion made from that experiment, something else other than the actual stimulus that was meant to produce those results in an experiment. This means these result may not be a true representation of the research outcome because something else has intervened and had it effect on the participants. Dependent variable are affected by something else outside of the experimental stimulus.

"participants get more benefit from the human contact of a research " this is something else other than the clinical treatment.