Over at Overcoming Bias, Robert Wiblin offers a tempting reward:

Personally, I would like to think I take doing the right thing
seriously, so I am willing to offer a monetary prize of £300 for anyone
who can change my mind on a) whether I ought to place a significant
probability on moral realism being correct, or b) help me see that I
seriously misunderstand what I subjectively value. Such insights would
be a bargain!

I’m happy to try my luck on (a).  Robert states the following reasons to doubt the truth of moral realism:

1. People put little effort into acquiring and refining moral knowledge:

If you really believed that there were objective rules that we should follow, that would make it crucial to
work out what those rules actually were. If you failed to pick the
right rules, you could spend your life doing things that were worthless,
or maybe even evil. And if those are the rules that everyone
necessarily ought to be following, nothing could be worse than failing
to follow them. If most acts or consequences are not the best, as seems
likely, then the chances of you stumbling on the right ones by chance
are very low.

2. People have inconsistent moral views:

Simple probing using
questions well known to philosophers usually reveals a great deal of
apparent inconsistency in people’s positions on moral issues. This has
been known for thousands of years, but we are scarcely more consistent
now than in the past.

3. People worry little about moral uncertainty:

A moral realist should also be trying to spread their bets to account for ‘moral uncertainty‘.
Even if you think you have the right moral code, there is always the
possibility you are mistaken and in fact a different set of rules are
correct. Unless you are extremely confident that the rules you consider
most likely, this ought to affect your behaviour.

Example:

Let’s say the number
of (post-)humans we expect to live in the future, in the absence of any
collapse, is a modest 1 trillion. The real number is probably much
larger. If you thought there were just a 10% chance that people who
weren’t alive now did in fact deserve moral consideration, that would
still mean collapse prevented the existence of 100 billion future people
in ‘expected value’ terms. This still dwarfs the importance of the 7
billion people alive today, and makes the case for focussing on such
threats many times more compelling than otherwise.

4. We haven’t evolved to grasp moral truth:

[T]here is no obvious reason for our moral intuitions to be tethered to
what is really right and wrong full stop. It is almost certain that
humans came about through the process of evolution. Evolution will give
us the ability to sense the physical world in order to be able to
respond to it, survive and reproduce. It will also give us good
intuitions about mathematics, insofar as that helps us make predictions
about the world around us, survive and reproduce. But why should natural
selection provide us with instinctive knowledge of objective moral
rules? There is no necessary reason for such knowledge to help a
creature survive – indeed, most popular moral theories are likely to do
the opposite.

The main problem with all of these arguments is that they prove far too much.  The same holds for philosophy, religion, politics, social theory, and cosmology.  Indeed, it holds for the science of global warming.  People put little effort into understanding climate change. (#1)  People worry little about climate change uncertainty. (#3)  People haven’t evolved to understand climate change, since humans in the ancestral environment had very little effect on global climate, and the change takes lifetimes. (#4) 

I’d even argue that people’s views about climate change are extremely inconsistent. (#2)  If you believe that lower demand for fossil fuels in in clean countries will reduce fossil fuel prices in dirty countries, why aren’t First World greens worried that reducing their carbon footprint will counter-productively increase carbon emissions in the Third World?

By Robert’s logic, these are all reasons to doubt that climate change is objective, or at least that humans have any knowledge of climate change.  But that’s an absurd leap.  These are all reasons to think harder about climate change, and wonder if what we think we know is right.  But none of them are reasons to think that climate change is all in our heads.  The same goes for philosophy, religion, politics, social theory, cosmology, and yes, morality.

Have I “proven” that moral realism is true?  Of course not.  But if Robert’s four stated reasons are why he doubts moral realism, he has no strong reason to think that morals are any less real than any other subject that excites human emotions.  Unless he’s a solipsist, then, he “ought to place a significant
probability on moral realism being correct.”  And he ought to pay me £300.

HT: Everything I know about meta-ethics I learned from Michael Huemer.