Holding a participant characteristic (such as age or gender) constant strengthens and weakens internal validity; external validity external validity; internal validity reliability; validity accuracy; reliability

The correct answer is internal validity; external validity.

1. Internal validity refers to a characteristic of a research study that is successful in eliminating confounding variables so that the results are obtained from manipulation of the independent variable(s) alone, and not any other confounding or extraneous variables. In this instance, by controlling variables, like age and gender, the internal validity of the study is strengthened. 

2. External validity refers to a characteristic of a research study wherein the findings can be generalized or extrapolated to other settings and other populations. In this instance, when variables like gender and age are controlled, the study is low in external validity because the findings can only be extrapolated to limited population, and not every gender and age group.