Now, ritual cannibalism has occurred in all cultures (Eating someone heart to get his strength for example) but true cannibalism, eating people for food, has never been found to have occurred except during times of extreme emergency (and these are NOT caused by Famine or something like that but man-made actions such as cannibalism reported during sieges of cities by enemy forces, the last time that occurred was during WWII and the siege of Leningrad, OR people cut off from other sources of food by their own actions, the Donnor Party of the Oregon Trail fame for example). As one writer says, during most famines the concern is NOT people eating people for the simple fact during famines everyone is starving at the same rate and it is easier to spend one's energy looking for what ever other food exists NOT to kill your fellow man. It is only when such searches for food is not possible that you see cannibalism occurring, such as sieges, or people cut off from supplies by winter or other disaster.
One person in the mid 20th century actually tried to find first had reports of cannibalism and all he found was report that some distant tribe over some distant mountain were cannibals. When he arrived at that tribe, he found similar reports often is the same direction i.e. away from the main trade routes. Cannibalism has occurred but never for any length of time (Rarely over a month as most sieges end about the same time as cannibalism starts to kick in) and then only in very limited numbers (The reports from Leningrad only number in the hundred, in a multiple million population, and were always mothers looking for food for their children and then always someone who was already dead).
Here is Wikipedia, which repeats a lot of the tales that "Evil" people resorted to Cannibalism (i.e. Soviet reports of German Soldiers at the Battle of Stalingrad, US and Australian Reports of Japanese doing the same, through some of this may be ritual cannibalism, the Japanese Army of WWII was into far right lunacy that makes today Right wing look absolutely sane, reports of use of Cannibalism as an intimidation tool by the Soviet Secret Police under Stalin etc). Read these reports carefully, most are questionable and keep in mind no one is reporting to having eaten anyone OR seeing anyone being eaten EXCEPT in cases of Ritual Cannibalism or an insane person.
http://en.wikipedia.org/wiki/CannibalismLeningrad and reports of Cannibalism:
http://books.google.com/books?id=jt8QVm8dPaQC&pg=PA239&lpg=PA239&dq=siege+of+leningrad+cannibalism&source=bl&ots=oYTOUBv9H1&sig=9_Z2meD7Y2AqTdzsJZufFbTBv_4&hl=en&ei=Rx8DS8jqK5LAlAf3x8XgAQ&sa=X&oi=book_result&ct=result&resnum=8&ved=0CCAQ6AEwBw#v=onepage&q=siege%20of%20leningrad%20cannibalism&f=false