There are two new books out about the psychological differences between liberals and conservatives, trying to connect these differences to genetic causes. The theory makes sense: a person's proclivity to feel disgust or his or her openness to new experiences could influence political ideology, and these psychological traits probably do have some genetic component. Time will tell whether the evidence really supports it.
But I would like to address another big component of ideology: whether a person thinks most often at a micro-level or at macro-level. Micro-level thinking is focused on individuals and narratives; macro-level thinking is society focused. Typically, the conservative ideology of individual responsibility matches up to the micro approach: the compelling story of the hardworking student who succeeded on his own; a diligent mom who worked two jobs; or the responsible gun-owner's right to self-defense. On the other hand, the liberal ideology matches up to the macro approach: the need for affirmative action to create equitable educational opportunities; the need for decent-paying low-skill jobs; or the danger created for society by the proliferation of guns.
In some senses, it is really about competing versions of justice. The micro-level is about just desserts: each person is rewarded or punished for his or her good or bad deeds. Of course it would not be acceptable, in this view, if the hardworking student is thwarted by a racist or other discriminatory policy; what matters that there is a fair opportunity to succeed. On the other hand, the macro view is about society-level outcomes; equal opportunity alone is not sufficient. For example, even if no students are actually discriminated against, the macro view would say an outcome where only 16% of the native-born African American population graduates college but 30% of native-born whites do (this is currently true in the U.S.) do is unjust. To paraphrase from Nozick, the macro view presumes a pattern to fit onto society, but the micro view merely looks at the justice of all the individual transactions -- and whatever overall pattern that happens to emerge from these transactions is morally neutral. Here is Mankiw giving a clear voice to this mode of thinking.
As a way of explaining political ideologies to debaters, I have found this idea of competing definitions of justice is quite helpful. It is simple to understand but not too simplified. To be sure, there are times when conservatives do show a more macro-level vision of justice, and liberals show a more micro-level vision, but these are exceptions to rule.
The best analogy I could think of is roads in an old city. If the process for deciding where to build the roads was fair, a micro-level view would have no problem with the end results, if even the road-by-road accretion over time resulted in some very odd configurations. The macro-level view would try to find ways to make a city's roads conform to a pattern, either by massive construction to relocate the roads or by trying to force a clearer design from the beginning. Think old European city twisty roads vs. midwestern city roads in a strict grid. (Click here for a neat visualization and compare London to Chicago.)
At times, each level of thinking is appropriate. It is certainly appropriate for judges' decisions in court cases to be made at the micro-level. On the other hand, it is right to look at patterns in the convictions; we should know what factors bias decisions. The society-level concerns cannot be rammed into individual cases, which would be unjust for many defendants, but the knowledge of biases should inform decisions. Unfortunately, I do not believe there is a way to be more systematic about it than to say "inform." The balancing act is tricky, and neither way of thinking should dominate.
Here are some examples to ponder:
Should utilities charge different rates based on usage? For example, should the first kWh a person purchases cost 8 cents, the second kWh cost 8.1 cents, ... the hundredth kWh 18 cents, ... and so on? At the individual level, this seems unfair. This would be asking different people to pay different overall rates: a small consumer would pay an average closer to 8 cents per kWh, while a large consumer might pay an average closer to 15 or 20 or even more cents per kWh. On the other, a pricing scheme such as this would serve two important societal goals: (1) it would allow cheaper access to electricity the smallest consumers, such as the poorest households; and (2) it would encourage conservation among the largest consumers. For example, high prices at the high end for major manufacturers might make investments in conservation worthwhile.
What should we do if we found out credit scores turn out to be a very good predictor of parolees' rates of recidivism? In other words, criminals with decent credit scores turn out to have a very low rate of committing another crime if they are paroled. Should we use this information in deciding parole? On the one hand, the promise of reducing recidivism serves a very important societal goal; on the other hand, the idea of using a person's credit score in this way seems unfair. Being bad at paying bills on time does not necessarily mean a person will commit a crime, so it seems more fair to make each determination on its own basis.
I have also written about teaching different ethical philosophies here.