the life expectancy increased from 74% in 1970 to 78%in 1990 what contributed to this factor
Life expectancy for whom? Newborns? Where? the United States? Denmark?
The question is too vague to Google.
Google something like this:
life expectancy infants United States
and you will get good articles, like this:
http://health.yahoo.com/news/178301
You can see the question you posed actually shows a drop in life expectancy as compared to the world.