Final answer:
Psychology's emphasis on evidence is the primary factor that makes it a science.
Step-by-step explanation:
The primary factor that makes psychology a science is its emphasis on evidence.
Psychologists use the scientific method to acquire knowledge and understand behavior. This method involves formulating hypotheses that can be tested through observations and experiments. It is a systematic approach that relies on empirical evidence, which is objective and observable.
For example, in a study on the effects of sleep deprivation on cognitive performance, psychologists would design an experiment, gather data, and analyze the results to determine if there is evidence to support their hypothesis. This emphasis on evidence distinguishes psychology as a science.