Final answer:
A high internal validity in an experiment indicates that the experimental manipulation directly causes any observed differences in the dependent variable, which affirms a causal relationship.
Step-by-step explanation:
An experiment has high internal validity if the design ensures that the manipulation is what caused a significant difference on the dependent variable. Internal validity is critical for establishing a cause-and-effect relationship between the independent and dependent variables. It involves the researcher designing the experiment so that the only difference between the experimental and control groups is the exposure to the experimental manipulation, thus controlling for lurking variables. This distinction is crucial in asserting that any observed effects on the dependent variable are indeed attributable to the manipulation of the independent variable, not to extraneous factors.
To achieve internal validity, an experiment must be set up with a strict control group and potentially utilize techniques such as random assignment to treatment groups to ensure that lurking variables are evenly distributed among the groups. When the data appraises a significant difference in the dependent variable across the groups, we can infer a cause-and-effect relationship - that is, the experimental manipulation is responsible for the alteration in the dependent variable.