Via the extropians list comes this psychology paper [google cache > rtf], neatly summarised by Anders Sandberg of aleph.se fame:

The basic idea is the cognitive science of sacred values. Something is sacred when it is absolute and involable, and the mere thought of trading it off for some secular value is abhorrent. Different societies have different sacredness, but all societies have it. In practice we always have to do trade-offs between values. As an example, how much do we want to pay for road safety? If human lives were infinitely valuable to us, we would gladly pay all money we could for safer roads. But we actually chose (or have someone chose for us) a finite amount. But actually being tempted or forced to make these compromises makes us uncomfortable, and various escape and coping mechanisms (such as ‘moral cleansing’ where people do something they consider moral to balance the trade - we feel contaminated by trading in the sacred). These taboo tradeoffs cause strong moral outrage, especially among observers. It is seen as unacceptable to trade something sacred for secular, or even to consider it.

Read the paper for actual data!