What type of research method involves an investigator manipulating variables to observe effects on behavior?

The correct answer is the experiment, which is characterized by the manipulation of one or more independent variables to observe the effects on a dependent variable. This method allows researchers to establish cause-and-effect relationships by controlling for extraneous variables and isolating the specific factors being studied. In an experimental design, participants are often randomly assigned to different conditions, which helps to minimize biases and increases the validity of the results.

Descriptive research, on the other hand, focuses on providing a detailed account of a phenomenon without manipulating any variables, merely observing and describing the characteristics or behaviors within a specific population. Causal-comparative research seeks to identify causal relationships by comparing groups that differ on one or more variables but does not involve direct manipulation of those variables by the researcher, making it less definitive in establishing causal links than experimentation. Correlation research examines the relationship between two variables but does not imply any causal direction, as it primarily assesses the strength and direction of a relationship without manipulation of the variables involved.