How does Karma work?

Does the Titanic sink because of the hubris of its builders, the misdeeds of the captain or the collected naughtiness of the passengers? Does each individual own their own slice of karma or is it shared and therefore corruptible by one bad apple? If it is in our nature to act in ways that might be perceived as negligent by others (for example) are we instantly folded in to the karmic cycle of negative returns? Some of these questions addressed indirectly here.