Latona did an experiment at a military barracks over the space of a few months to examine the effect of group size on group morale. He randomly assigned soldiers to the experimental and control groups and did a pretest and posttest. Midway through the experiment, there was an incident in a nearby city involving some soldiers in which the soldiers were visibly mistreated by the community, which affected the morale of both the experimental and control groups. Which source of internal invalidity does this example reflect?

Internal Invalidity refers to the fact that the outcome of an experiment may not be the correct representation results of the actual experiment which means something else (other than the stimulus that was meant to affect the experiment ) affected the experiment so this thing tend to manipulate the dependent variable. These are sources of internal invalidity :history , testing,statistical regression, maturation, instrumentations,demoralization and experimental mortality.

Some historic event may impact the experiment and it's result apart from the experiment stimulus that is meant to impact the results. Mistreatment of soldiers by the community is a historic event that will now impact how the soldiers will respond to the experiments without Latona's input or effect on the experiment.