I am an only child, are there any problems associated with being raised that way?
no not really.  some children have half-siblings or step-siblings living at the same house or a different house may have a similar family environment to only children.
Are there a lot of only children IN the United States?  I feel like other countries usually hyave lots of kids?
only children were relativily uncommon throughout history 
Why is that?  I would think having less mouths to feed would be a good thing?
well starting at around the 20th century, birth rates and average family sizes started to fall sharply due to the costs of raising children and women having their first child later in life.
Yeah I guess that makes sense.  Women are more involved now in the workplace, so they'll want kids later as well.
Starting at around the great depression the proportion of families in the US with only children increased
The Great Depression changed a lot of things, I wouldn't have considered family size to be one of them!
its also known that around 1953 after the korean war ended, the south korean government suggested citizens have one or two children only to boost economic prosperity