In this article , Rebecca Saxe reviews three areas of experimental psychological research bearing on questions of moral psychology.  The first are the various experiments, conducted by Marc Hauser and others, soliciting individuals’ responses to Foot’s trolley problem.  The results are surprisingly consistent across the various genders, cultures, etc., lending apparent support for the existence of a universal moral instinct.   Psychologists in this area have  hypothesized that the universality of this moral instinct is akin to the universal facility for natural language. Saxe:

"The current theorists take as their
model for moral reasoning not conscious problem-solving, as Kohlberg
did, but the human language faculty. That is, rather than “moral
reasoning,” human beings are understood to be endowed with a
“moral instinct” that enables them to categorize and judge
actions as right or wrong the way native speakers intuitively
recognize sentences as grammatical or ungrammatical.
… We can draw
three predictions from the theory that morality operates as language
does. First, just as each speaker can produce and understand an
infinite number of completely original sentences, every moral
reasoner can make fluent, confident, and compelling moral judgments
about an infinite number of unique cases, including ones that they
have never imagined confronting. Second, cross-culturally, systems of
moral reasoning can be as diverse as human languages are, without
precluding that a universal system of rules, derived from our
biological inheritance, underlies and governs all these surface-level
differences. Finally, just as native speakers are often unable to
articulate the rules of grammar that they obey when speaking, the
practitioners of moral judgment may have great difficulty
articulating the principles that inform their judgments."

The first prediction seems to be unlikely, the third very likely, and the second murky: In what sense can there be a "universal system of rules" underlying our moral judgments consistent with the apparent diversity of moral reasoning and moral verdicts?  Here it seems to me the analogy with language breaks down: The universal principles of natural language syntax are realized in numerous natural languages, but it’s tougher to see how the numerous moral outlooks human beings exhibit is a realization of some underlying system of rules instead of a realization of a weaker universal tendency to apply moral concepts.  There can certainly be a universal moral instinct without a universal set of substantive moral principles that governs our moral psychology.

The second area of research Saxe discusses are the studies concerning the moral-conventional studies, instigated by Elliott Turiel.  Infant studies suggest that children make a moral-conventional distinction (i.e., a distinction between norms that exist independent of authority or convention, and those norms (such as etiquette) that do not) fairly early in life, and can make simple distinctions between helping and harming behavior.

Finally, Saxe describes the work done with brain imaging in an attempt to physically locate the brain regions responsible for moral judgments.

Fascinating stuff!