Answer:
experimental method
Step-by-step explanation:
Experimental method will not be possible because under this method, he will have to induce the test behaviour on the subject, which in the case is spanking. This will be unnecessary subjecting children to pain, this is ethically wrong. Even in animal experimental, physical pain and cruelty is not allowed.
Naturalistic observation and case study methods will be better for such a study. Correlation of reported past experiences and outcomes in individuals who suffered spanking is also a good approach.