Modern Ethical Theories

Philosophically Egoism theory purports that one’s self is or should be the motivation and reasons of our own actions.  This theory has two variations, descriptive or normative variation and altruism variation.  Altruism variation holds that one should maximize good for all being in the world (Hobbes, 1651).  Altruism is the opposite of egoism.
Ethical egoism is a normative theory that revolves around the idea that each individual should do what is entirely in his or her self interests and by doing actions that aimed at self interest he or she achieves morality.  The theory states that one’s self interest is the primary motive of actions and is the moral thing to do. Hence the theory is a self effacing theory which tries to justify reasons for actions as duty bound or morally right.
Psychological egoism theory on the other hand is a descriptive theory that describes human actions as wholly motivated by self interest it implies that individual’s actions are purely by self centered even though the actions may seem to be motivated by interest of others (More, G.E., 1903).  It states that, “all human actions are selfish acts at their roots”.

Thus it asserts that if individual helps others, acts which may seem to be of benefit to others.  The primary motive may be the need for an individual to better him/herself thus making the act self-seeking.  It is impossible to empirically prove the psychological egoism theory wrong but it is just as hard to empirically prove the theory right.  In addition once the premise of the theory is accepted the actions can be interpreted to support the theory.
Fallacy of Psychological Egoism
Psychological egoism theory states that people are always motivated by their own interests.  This  however hastily  generalizes actions and their motivation  since there are instances we can disapprove this such as, when an individual smokes .this is an act that can be harmful to one’s wellbeing thus not act in  self interest, others such as soldiers carry out their duties without considering personal gains. There are also instances where motivations to act may appear to be of no self interest  at face value but  are nevertheless self interest motivated (Rachels, J., 2003).  For example if an individual is motivated to act out of fear or his acts are motivated by fear, he/she may be acting in the interest of self-preservation.
On the other hand, these actions/reactions may be driven by the interest of the other individual or threats from this other individual thus his/her action may be classified as an action motivated for other’s interest.  In this case, the individual actions are what she/he is motivated to do, for instance run.
The strong and weak versions of Ethical Egoism
The ethical egoism can be split into two versions; the strong and the weak versions.  The strong version supports the endorsement of an individual’s own good as the moral thing to do.  Individual actions that are driven by self-interest or self fulfillment arise out of moral judgement while the weak version supports the premise that there are situations where it may be prudent to ignore personal welfare when making moral judgement (Rachels, J., 2003).
For instance if a soldier lies on a grenade to save his friends he may act this way to be able to live with himself In other words if he acted otherwise he may live the rest of his life with a guilty conscience .However the strong version might interpret this action as the moral thing to do.
Contrast in Motivation Doctrines of the two theories
Psychological egoism theory is the empirical doctrine that an individual’s act is a desire for one’s own welfare (Sigmund, F., 1966).  However, these acts usually conceal the determining motives.  This concealment is usually in their self-interest.  The theory results from observation of human behaviour and can only be proved empirically if there are no exceptions.  It makes no claim to how individuals should act for their self-interest.  This premise can be argued to be verifiable but non-moral.
On the other hand, the ethical egoism theory is prescriptive doctrine that individuals actions are in self-interest that is for personal welfare thus the personal welfare is the most valuable thing for an individual.  However, the theory does not claim that all individuals seek their own self interest.  It only claims that individuals should seek their own self-interest that is all individual might not seek self-interest (Sigmund, F., 1966).  For the ethical egoism theory to hold or to be proven as a theory its premises must hold for all individuals.
Differences between ethical egoism and psychological egoism
Psychological egoism is descriptive while the ethical egoism theory is prescriptive, that is, the psychological theory states why individuals act the way they do while the ethical theory suggests the reasons for such.  The other difference is concerned with their motivation doctrines.  In psychological egoism it is human nature to be driven by selfish reasons whereas in the ethical egoism theory selfish acts are the morally right thing to do (More, G.E., 1903).Psychological egoism deals with reality while ethical egoism deals with what reality should be. It  can also be argued that ethical egoism approves of cooperative behaviour  while psychological egoism does not since the theory  believes in achieving maximum welfare though selfish actions .
Conclusion
Self-interest actions are not always selfish actions in that one can act in such a way as to benefit or for his personal welfare improvement.  Such actions cannot be termed as selfish and this is always seen as acts out of self-interest.  Self-interest acts are not always selfish acts but selfish actions are always out of personal self-interest.
For example if an individual’s self interest is to obey the law we can claim the individual is doing this out of self interest, that is, it is in his/her self interest to avoid legal troubles like being jailed.  This can be seen as a selfish act.  But by obeying the law, it can also be argued that he is doing this in the interest of others or out of consideration of others.  In this case, this is not a selfish act.  However, self-interest actions are not always incompatible with the interest of others for example to help others, which is not a self-interest act.  An individual must first help himself inorder to be in a position to help others.
Reference
G.E. Moore (1903).  In Principia Ethica: Cambridge University Press.
Rachels, J. (2003). The Elements of moral philosophy (4th ed). New York: McGraw-Hill.
Sigmund, Freud (1966).  The Interpretation of Dreams.  New York: Avon.