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!
to the country.
From 1979 to 2015, the one-child policy in the People's Republic of China restricted most parents to having only one child, although it was subject to local relaxations and individual circumstances (for instance when twins were conceived).
Families may have an only child for a variety of reasons, including: personal preference, family planning, financial and emotional or physical health issues, desire to travel, stress in the family, educational advantages, late marriage, stability, focus, time constraints
Only child
An only child is a person with no siblings, either biological or adopted. 
Children who have half-siblings or step-siblings, either living at the same house or a different house - especially those who were born considerably later - may have a similar family environment to only children, as may children who have much younger siblings from both of the same parents (generally ten or more years).
Overview.
Throughout history, only children were relatively uncommon. From around the middle of the