The relationship between Parfit’s theory of Personal Identity and his research on Population Ethics is underexplored. In this paper I both examine this relationship and support the principle stating that it is moral to cause the greatest total wellbeing. Once introduced the basic concepts of Population Ethics and Parfit’s theory of Personal Identity, I report Parfit’s distinction between Different Number Choices, that affect both the number and the identity of future people, and Same Number Choices, that affect only future people’s identity. Parfit underlines how, in Different Number Choices, it is doubtful whether someone ought to cause a higher average or a higher total of good. Assuming Parfit’s theory of Personal Identity I argue that personal choices such as choosing between possible careers determine the chooser’s future identity: such choices are thus Same Number Choices. Since personal choices can influence a life’s length and enjoyed wellbeing, Same Number Choices share with Different Number Choices the dilemma of choosing between higher average and higher total of good. In the final part of the paper I support the principle of total good by suggesting that the crucial objection against it, called Quality Condition, seems to be valid only for populations too big to be correctly compared and violates a rule that seems obvious in small populations. The equivalent of the Quality Condition in Same Number Choices seems to be valid only for amounts of times too big to be correctly compared and violates an obvious prudential rule in short times.