It would be impossible to scientifically predict human behavior. We can study behavior from a biosocial, biological, psychological, and social perspective, but the ultimate act of decision-making is not something that can be pinned down as an expected course of action. It is also impossible to assume that people take the same course of action under a specific, equal, circumstance because the process of choice-making is inherent to individual personality traits. Furthermore, it is not possible to apply the scientific method to the study of behavior because equal variables will hardly replicate over and over in the exact same fashion to establish a correlation.
What we can do, however, is to pay close attention to the neurological and psychological processes that take place in during motivation. It is motivation that ultimately dictates action. Therefore, in order to make a choice as big as forgiving, we must keep in mind that the individual MUST have made a correlation between the act of forgiveness and a motivating factor that will occur as a result of such forgiveness.
Let's look at the example of Nelson Mandela in 1990.
After an interminable imprisonment, he is quoted as having forgiven the people who put him in that situation and saying:
“We especially need to forgive each other, because when you intend to forgive, you heal part of the pain, but when you forgive you heal completely.”
Regardless of the moral aspect of forgiveness (as morality is also viewed differently from one group to another), consider that there is a motivating factor behind Mandela's choice to forgive: healing.
His independent, unique, and individual neurological and emotional processes elicit the belief that he can heal as a result of forgiveness. It is not a scientific process, but a trait-based choice that he made as a result of his particular circumstances.
Therefore, to try and apply a scientific methodology to the study of behavior would be to deny that all humans are the same merely in terms of biological make up. When it comes to personality traits, spirituality, morality, psychology, emotional intelligence, and maturity of character each of us is painted with a completely different brush.