If someone believes something about the moral implications of conception, that is something they likely just took in as a social truth, and then later learned and crafted a rationalization for. I don't think we have moral instincts about cellular organisms.

To the extent that their in group remains constant, it would take a lot of serious moral argument to overcome that social truth. The problem with political arguments is that people don't seriously have them. They volley a couple of bumper stickers at each other, and then go off in a huff. They may do it a million times - but a million bogus arguments designed to achieve nothing individually will likely achieve nothing in the aggregate as well.

The social truth remains intact - no serious moral arguments oppose it - why would we ever expect a change?

Where you might expect a change - when one moves between social groups, or when one commits to and is capable of serious moral argument.

The social aspect is at least theoretically testable - how big are the moral shifts when people change in groups? I suspect pretty large.

And in the relatively rare ideologue class, the shifts are often pretty big too. I knew a gal who went from seminary->Leninism->WIcca/EnviroProgressivism. Lots of atheist ideologues are former fundamentalists. Conservatives who were former marxists.

Arguments work on people who engage in them. I'd guess that a changing social truth works even better on most.