Answer:
earlier i was concerned about your crucial expression
They are fighting because they are rivals.
Answer:
There is no systematic research only bits and pieces. Animal researchers have determined that some animals have a sense of justice (some monkey species will reject a treat if they see you give a better treat to another monkey), feel love and jealousy (dogs; the former from brain scan proof), feel anger, target specific humans for revenge (some birds), and thus that moral sense evolved.
Brain scans on humans making ethical choices in conditions of ethical conflict show that two brain regions, one akin to disgust and one to caluclating effects, are involved; people with brain injuries to the former (and psychopaths) calculate only using the latter. (Look up “the Trolley Hypothetical” online).
Earlier research about the stages of moral development have been debunked. Lawrence Kohlberg's stages of moral development - Wikipedia
Essentially anything affecting the human brain and human decision-making and human personality can also affect moral reasoning and the behavior choices related to it. Thus studies of criminals have shown that each additional bad thing increases the risk of later criminality, including being rejected by your Mom at birth, a difficult birth, Mom’s pre-natal drug/alcohol use, drug/alcohol abuse in the home, being a victim of or seeing violence, malnutrition, etc. Thus moral choices can have many influences.
When a person pays their debts, one can determine that the person is responsible and able. They become more trustworthy and reliable.
