Which is a worldwide trend in the standard of living since the mid-twentieth century?
The life expectancy in developed countries fell from 74 to 66 years.
The life expectancy in developing countries rose from 40 to 61 years.
The literacy rate in sub-Saharan Africa decreased from 50 percent to 12 percent.
The literacy rate in China increased from 5 percent to 100 percent.

The life expectancy in developing countries rose from 40 to 61 years.

Explanation:

Currently the life expectancy of developing countries excluding China is around 67-71 years. The life expectancy of the poorest nations (least developed countries) is slightly lower, around 63-66 years). Even though this numbers are significantly lower than the life expectancy in developed countries (76-82 years) it represents a large increase since the 1950s.

During the 1950s, the life expectancy in the least developed countries was around 40 years, and for the rest of developing countries it barely reached 50 years. E.g. life expectancy in Mali (Africa's lowest) was 26.4 years in 1950, now it is 59.3 years. In China it was 41 years, now it is 76.9 years.

This increase in life expectancy shows that the standard of living has improved around the world, even in poor countries. Some developing countries have showed a significant increase in life expectancy along with a significant increase in economic development, e.g. South Korea's life expectancy was 35.6 years and now it is 83 years.

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