Lucy Burroughs answered
As thoughts and feelings are not really quantifiable - meaning you can't count them - it's very difficult to measure them scientifically. The closest we can get to studying thoughts and emotions is by approaching psychology as a science.
A scientific approach to psychology might be interested in the relationship between brain structure and aggression, and would use scientific equipment such as a brain scanner to achieve this.
Psychology as a Science
Some branches of psychology (such as biological, which looks at brain chemistry and the like) and developmental (which looks at how the child's brain develops) seek to explain human thought and behavior by approaching it from a scientific perspective.
Here are some of the methods used to make the studies scientific:
- Control Over Variables - This means making sure that the only thing affecting the results is the thing being measured. For example, a study on the effects of caffeine on memory recall might ensure that all participants were given the same amount of caffeine, had slept for the same amount of time the night before, and were allowed to look at the list of words for the same amount of time.
- Empirical Evidence - This means that the data collected from the study is accurate and quantitative. This can be achieved through the use of brain scans, pulse monitors, and the like.
- Objectivity - Researchers often do what is known as a 'blind study' to ensure that their personal opinions, predictions and biases don't affect the results. This could be achieved, for example, by using two group conditions (one given caffeine and one given nothing), but without the researcher knowing which group is which when they're looking over the data. (They can check afterwards, though!)
- Replicability - For all scientists and many psychologists, it's important that a study or experiment can be repeated and still produce the same results. This means that the experiment must leave no room for extraneous variables (things with the potential to affect the results, other than the thing that they want to test).