If the left takes account of evolutionary psychology, Singer argues, it will be better able to harness that understanding of human nature to implement policies which have a better chance of success. In doing so, two evolutionary fallacies have to be cleared up. First of all, we have evolved not to be ruthless proto-capitalists, but to "enter into mutually beneficial forms of co-operation." It is the evolutionary psychologist’s work in explaining how ‘survival of the fittest’ translates into co-operative behaviour which has been, arguably, its greatest success. Secondly, there is the "is/ought" gap. To say a certain type of behaviour has evolved is not to say it is morally right. To accept a need to understand how our minds evolved is not to endorse every human trait with an evolutionary origin.
On the origin of ethics:
Singer believes Darwinian theory gives us an understanding of the origin of ethics, because, for example, it gives an evolutionary explanation of how reciprocity came to be. Put crudely, if you model the survival prospects for different kinds of creatures with different ways of interacting with others - from serial exploiters to serial co-operators and every shade in between - it turns out that the creatures who thrive in the long run are those that adopt a strategy called ‘tit for tat’. This means that they always seek to co-operate with others, but withdraw that co-operation as soon as they are taken advantage of. Because this is the attitude which increases the survival value of a species, it would seem to follow that humans have evolved an in-built tendency to co-operation, along with a tendency to withdraw that co-operation if exploited. Hence, it is argued, and essential feature of ethics - reciprocity - is explained by evolution.
It's about constraints:
I put it to Singer that, it follows that the moral judgements that we’re going to make are going to be of the sort, ‘If the evolved behaviour is going to lead to the morally desirable result follow it and if the evolved behaviour does not lead to the morally desirable result, don’t follow it’. So isn’t the observation of what has evolved going to drop out of the equation? It’s not going to feed at all directly into what our moral rules are going to be.Singer’s answer reveals more precisely the limited, but important role, he believes Darwinian explanations play in our ethics. "I think the Darwinian is going to alert us to what rules are going to work and what rules are going to meet a lot of resistance and I think we have to bear that in mind. But always there’s a trade off between how important the values are to us and the strength of the evolved tendency in our natures."