I'm sure you've seen this at some point, but for others...
On Twin Earth, a brain in a vat is at the wheel of a runaway trolley. There are only two options that the brain can take: the right side of the fork in the track or the left side of the fork. There is no way in sight of derailing or stopping the trolley and the brain is aware of this, for the brain knows trolleys. The brain is causally hooked up to the trolley such that the brain can determine the course which the trolley will take.
On the right side of the track there is a single railroad worker, Jones,who will definitely be killed if the brain steers the trolley to the right. If the railman on the right lives, he will go on to kill five men for the sake of killing them, but in doing so will inadvertently save the lives of thirty orphans (one of the five men he will kill is planning to destroy a bridge that the orphan's bus will be crossing later that night). One of the orphans that will be killed would have grown up to become a tyrant who would make good utilitarian men do bad things. Another of the orphans would grow up to become G.E.M. Anscombe, while a third would invent the pop-top can.
If the brain in the vat chooses the left side of the track, the trolley will definitely hit and kill a railman on the left side of the track, "Leftie" and will hit and destroy ten beating hearts on the track that could (and would) have been transplanted into ten patients in the local hospital that will die without donor hearts. These are the only hearts available, and the brain is aware of this, for the brain knows hearts. If the railman on the left side of the track lives, he too will kill five men, in fact the same five that the railman on the right would kill. However, "Leftie" will kill the five as an unintended consequence of saving ten men: he will inadvertently kill the five men rushing the ten hearts to the local hospital for transplantation. A further result of "Leftie's" act would be that the busload of orphans will be spared. Among the five men killed by "Leftie" are both the man responsible for putting the brain at the controls of the trolley, and the author of this example. If the ten hearts and "Leftie" are killed by the trolley, the ten prospective heart-transplant patients will die and their kidneys will be used to save the lives of twenty kidney-transplant patients, one of whom will grow up to cure cancer, and one of whom will grow up to be Hitler. There are other kidneys and dialysis machines available, however the brain does not know kidneys, and this is not a factor.
Assume that the brain's choice, whatever it turns out to be, will serve as an example to other brains-in-vats and so the effects of his decision will be amplified. Also assume that if the brain chooses the right side of the fork, an unjust war free of war crimes will ensue, while if the brain chooses the left fork, a just war fraught with war crimes will result. Furthermore, there is an intermittently active Cartesian demon deceiving the brain in such a manner that the brain is never sure if it is being deceived.
QUESTION: What should the brain do?
[ALTERNATIVE EXAMPLE: Same as above, except the brain has had a commissurotomy, and the left half of the brain is a consequentialist and the right side is an absolutist.]
Choose the left track, because cancer kills more people than Hitler (assuming the cure would be delayed by at least 10 years, implementing it doesn't cost more than is currently spent on cancer and a few other things).
The trolley problem is one of the more famous thought experiments in moral philosophy, and studies by psychologists and anthropologists suggest that the response distributions to its major permutations remain roughly the same throughout all human cultures. Most people will permit pulling the lever to redirect the trolley so that it will kill one person rather than five, but will balk at pushing one fat person in front of the trolley to save the five if that is the only available option of stopping it.
However, in informal settings, where the dilemma is posed by a peer rather than a teacher or researcher, it has been my observation that there is another major category which accounts for a significant proportion of respondents' answers. Rather than choosing to flip the switch, push the fat man, or remain passive, many people will reject the question outright. They will attack the improbability of the premise, attempt to invent third options, or appeal to their emotional state in the provided scenario ("I would be too panicked to do anything",) or some combination of the above, in order to opt out of answering the question on its own terms.
However, in most cases, these excuses are not their true rejection. Those who tried to find third options or appeal to their emotional state will continue to reject the dilemma even when it is posed in its most inconvenient possible forms, where they have the time to collect themselves and make a reasoned choice, but no possibility of implementing alternative solutions.
Those who appealed to the unlikelihood of the scenario might appear to have the stronger objection; after all, the trolley dilemma is extremely improbable, and more inconvenient permutations of the problem might appear even less probable. However, trolleylike dilemmas are actually quite common in real life, when you take the scenario not as a case where only two options are available, but as a metaphor for any situation where all the available choices have negative repercussions, and attempting to optimize the outcome demands increased complicity in the dilemma. This method of framing the problem also tends not to cause people to reverse their rejections.
Ultimately, when provided with optimally inconvenient and general forms of the dilemma, most of those who rejected the question will continue to make excuses to avoid answering the question on its own terms. They will insist that there must be superior alternatives, that external circumstances will absolve them from having to make a choice, or simply that they have no responsibility to address an artificial moral dilemma.
When the respondents feel that they can possibly opt out of answering the question, the implications of the trolley problem become even more unnerving than the results from past studies suggest. It appears that we live in a world where not only will most people refuse complicity in a disaster in order to save more lives, but where many people reject outright the idea that they should have any considered set of moral standards for making hard choices at all. They have placed themselves in a reality too accommodating of their preferences to force them to have a system for dealing with situations with no ideal outcomes.