Approval Rate and Life Expectancy- The approval rate and life expectance show you the potential nations have for future population growth in their cites, as high numbers in both these categories means that there is plenty of room to added pop points to cities without having unhappy or unhealthy people. This is especially true in the early part of the game. Numbers around 50% mean they have reached the limit, while numbers below 50% show that they have overstretched their resources and could be vulnerable. Also if you look at the at the food yield along with the happiness and healthiness of a nation and find all three are high, especially in the later part of the game then it’s a good bet they are focusing on having specialists in their cities.