Final answer:
The internal validity of an experiment would be most harmed when participants who own a fidget spinner are not randomly assigned to conditions because personal characteristics related to ownership could confound the results.
Step-by-step explanation:
In an experimental setting where participants who own a fidget spinner are automatically assigned to the Spinner condition while those who don't are assigned to the No Spinner condition, the aspect of experimental design that is most compromised is internal validity. Internal validity refers to how well an experiment is done, particularly whether it avoids confounding (i.e., ensures that the only difference between the various groups is the factor being tested). In this scenario, since participants are not randomly assigned to conditions, personal characteristics related to fidget spinner ownership such as socioeconomic status or personality traits could influence the experimental outcomes, thus confounding the results.
Construct validity involves whether the experiment truly measures what it purports to measure and external validity relates to how well the findings can be generalized to the broader population. Participant reliability is not a commonly used term in experimental design; however, it could be interpreted as how consistently a participant responds within the context of the experiment, which is irrelevant to the method of assigning participants to conditions. Therefore, the most harmed aspect would be internal validity because the self-selection into groups could introduce lurking variables that were not controlled for, challenging the attribution of any observed effects to the presence or absence of the fidget spinner.