[TalonDJ]

But admitting the existence of a conscience would tend to support those who argue that children are born with an innate nature to do good and thus weaken the argument that children are born with a sinful nature.

Not at all. The Bible is very clear that we are born with both. The sin nature does not mean we fail to recognize good. Our conscience shows of the moral path from a very early age. Our sin nature means we are incapable of following the moral path all of the time. Of course even the definition and extent of 'sin nature' varies between various denominations. But sin nature in no way is contradictory with an inborn conscience.

[MrB]

That “contradiction”, IMO, is what refutes the evolutionary angle of the development of the conscience.

If doing good for the group (ie, the horizontal commandments) is an evolved survival mechanism,

why did we not also evolve a desire to do good?

This is what Lewis states is the “internal revelation” of God.