•explain the effects of cultural diversity among the american people during the late 20th and early 21st centuries.

That depends on which cultural diversity you are addressing. 
 The main one it would seem during the 20th century was prejudice between whites and blacks. Back then there was something called segregation, this word is used to describe the separation between black people and white people. Black people were not permitted to even use the same bathroom as a white person, and if there were not enough seats on a bus and a white person walked on, the black person would have to give up their seat for the white person. The government wanted to end segregation, so they made white children go far away out of town to a black school to be with blacks, and black children go far way from their town to a white school, this was a socialist experiment and it honestly did no good. Black people were treated poorly until people like Rosa Parks and Martin Luther King Jr. started standing up and fighting against this nasty prejudice. What those honorable men and woman fought for that cost them much, has no been obviously abundant in our lives. Although racism has not ended, we can see that segregation has. 

They most like influence us an there self