Final answer:
Most developed nations have a substantial middle class, representing an equal distribution of income and higher standards of living.
Step-by-step explanation:
The characteristic shared by most developed nations is a substantial middle class. This means that in these nations, there is a significant portion of the population that falls between the high-income and low-income groups, representing a relatively equal distribution of income and higher standards of living. Examples of developed nations with a substantial middle class include Canada, Japan, and Australia.