Maybe a little bit of a light-hearted question, but I find it curious as to how much peanut butter is a part of the culture of America, but not elsewhere.
1.) Why peanut butter?
2.) Where did the peanut butter & jelly sandwich come from? Why did it take hold in America so much but not elsewhere?
3.) Did it have anything to do with the advent of white, sliced bread and the disappearance of the local baker?
Actually the ubiquitousness of peanut butter in American culture is due in large part to several people:
The first one is Dr. John Harvey Kellogg. He ran a highly respected Michigan sanitarium in the late 19th century. He had a lot of theories about nutrition and health, some of which have been proven correct by modern medicine and others which are considered really strange today. He was very well-known in his time, treated many wealthy/powerful people, and his ideas were quite influential in American culture. He believed nuts were extremely healthy, and patented a process for making peanut butter out of steamed peanuts. He was devoutly Seventh Day Adventist, and as such he was a strict vegetarian. He marketed peanut butter quite widely as a healthy alternative protein source for vegetarians and also for elderly toothless patients who could not chew meat. He certainly did not invent peanut butter but he popularized it as a healthy food. At the time it was actually quite expensive, and mostly popular the among upper class people who frequented his sanatorium.
The second guy: George Washington Carver encouraged southern famers to grow peanuts when the boll weevil devastated cotton production in the south at the turn of the century. Carver improved the growability of the peanut plant and invented hundreds of uses for it. Many southern farmers turned to growing peanuts, which created a glut in the market. The price of peanuts began to drop, and many poor southerners started consuming them because they were cheap and nutritious.
The third guy would be George Bayle, a St. Louis businessman who began selling peanut butter as a snack food around this time. He sold tons of it during the 1904 St. Louis World's fair; it was such a hit that Heinz began selling it nationally soon after.
As far as PB&J sandwiches, the earliest recipes for them come from the same period, the turn of the century. Manufacturers in the 1920's hydrogenated peanut butter for shelf stability and began to add sugar, which made it appealing to children. Sliced bread was developed around the same time. Sliced bread meant that children could make sandwiches themselves without using a potentially dangerous knife. As a consequence of low cost, high nutrition, and ease of assembling, peanut butter sandwiches became popular as a child-friendly food during the depression.
This information comes from the book Creamy and Crunchy. Very informative, I read it a few months ago.
Everyone checking in from around the globe with the status of peanut butter in your respective countries, you do not need to do this and should stop (particularly those of you from the Netherlands, who apparently love the stuff). Unless you are willing to write a comprehensive and informative comment on the history non-American peanut butter popularity, saying "I'm from Place, and we Placians eat peanut butter all the time" is just so much noise. The OP has specified 3 distinct questions, please respect them by answering those queries.
I have another related question: most of the rest of the world enjoys savory peanut-based sauces, but they're (as far as I know) completely unknown in modern American cooking outside of "ethnic" cuisine. Is there any reason why this is the case? Were such sauces/condiments ever popular in the US?
Can I ask an additional question? What is the history of nutty spreads? Were there other nut-based spreads before peanut butter (although I assume there's also tahini, but I don't know its origins)?