Is the proverb "Ignorance is bliss" a fallacy? It's a questionable point, certainly. The study Sigelman mentions suggests that this is the case; that people are happier the more they know. I think this is certainly possible, as the more you know, the less likely you are to run into stressful situations through ignorance. There are many examples of cases where a little more knowledge would  have averted a great deal of unhappiness.

   My favorite illustration of the point occurs in Jane Austen's Pride and Prejudice. The Bennet family is ignorant of George Wickham's true character, which leads them into the error of permitting the flighty youngest daughter to accompany Wickham's regiment to its station in fashionable Brighton. Wickham takes advantage of Lydia's innocence and elopes with her, and the Bennets, their friends, and their relations suffer a great deal before the situation, easily prevented with a little more knowledge, is remedied. The entire family, with their friends and relations, would all have been much happier had they been a little better informed.

   In my own experience there have been several incidents when a little less ignorance would have materially contributed to happiness. In one case, it was the middle of winter and extremely cold outside. Two of our goats were pregnant, but we didn't know when they were due. It was through sheer luck that I discovered the newborns as I was checking on the sheep before bed; had they been born half an hour later, the kids would have died before morning. As it was, their legs and ears were frostbitten, and their bodies weak with cold. I stayed up all night with those kids, warming them in the house and trying to get them to eat. It was several days before they could walk properly, and their ears were always puckered from the frostbitten edges. Had we only known they were due, a close watch would have been kept on the mother, and the kids would have been born in a warm room, with us standing by to make sure all went well.

To use a historical example, had the British soldiers garrisoning Fort Ticonderoga been aware of the American plan to take the fortress, they would have made sure the fort was better guarded and in better repair. But the Americans found a weak point in the wall and snuck in, and Ethan Allen and his Green Mountain Boys took the fort with ease. If the British had known what was coming, they might have at least been fully dressed when they were taken prisoner.

There are many more examples, and they all support the point: you will be much happier if you know what's going on. Ignorance leads to a number of states, none of them even remotely blissful: they include stress, mess, injury, illness, defeat, and humiliation, to name a few. A stitch in time saves nine, but only if you've spotted the tear.

Edited a little when I typed it out, but not as much as I'd have liked. It took five minutes more than allowed on the test to write it. -_-;