Count the number of generations back to an ancestor who lived 100 years ago. Going back in time, you'll find that people started families younger. It is not uncommon for some to find five generations in the past 100 years, although four will be an average for this example. Conversely, your future generations will wait longer to have children; therefore, expect only three generations in the next 100 years.

Estimate the number of children. Looking back to your ancestor, find out approximately how many children each family had. For this example, use an average of 10 children for the first generation, five children for the second generation and three children for the third and fourth generation. For your generation and going into the future, two children for each generation would be average.

Calculate the data by multiplying the number of children by themselves for each generation. Use this format because you expect each child to have children for all successive generations. Using the average of four generations for the past 100 years, you would calculate as 10 children X 5 children X 3 children X 3 children = 450 descendants. This is many more descendants than the calculation for the next 100 years with three generations. Calculate this as 2 children X 2 children X 2 children = 8 descendants you will have in 100 years. The differences between your ancestor's descendants and your descendants is 442 people, lowering the population quite a bit over the next 100 years.