This study was completed at Cardiff University with 97 students. These students were asked to recall either a short sequence of letters or identify a missing item in a list. This memory test was completed on the independent variable of chewing gum. The control group did not receive chewing gum. The dependent variable is measured by the ability of memory recall. The study found that students who chewed gum performed significantly worse and researchers speculate that the tongue, mouth, and jaw movements of gum chewing disrupt short-term memory.
I do not believe this is a reliable study. The only subjects in the experiment were from this particular university and there are no specific details to their mental capabilities prior or how they were selected, so this experiment lacks a random sample needed in an experiment. All participants are from the same geographical region so the study lacks external validity. This study can not be generalized to all people. The researchers also do not specify what they were looking for as far as reaction speed or define what is "worse" in terms of testing to the control group who did not chew gum.
I would assume there are various other components that would contribute to memory in a short-term memory test, such as the amount of sleep, nutrition and physical activity each subject received for the days before the test was taken. This shows a lack of internal validity with the lack of eliminating third variables. Due to the lack of many necessary components of a reliable experiment we cannot infer causation that gum chewing actually hinders memory recall.