The simplest model is one first described Lotka in the 1920's and
formalized in the 1940's by Leslie. It is based on age-specific
survival and fecundity rates.^{10}

We take as the number of newly-born individuals at time . Thus

where is the maximum age to which individuals can survive. The number of individuals in other age categories is determined purely by the number of individuals that survive from the preceding year. Specifically,

This completely specifies the demographics of the population, assuming for the moment that the and don't vary from one year to the next. This can be written in matrix form as

More compactly

This model is usually referred to as a Leslie matrix model. It's important properties (as far as we're concerned) are:

- All yearly age classes are identified, each with their own
age-specific survival and fecundity rates.
- All members of a year class have the same probability of
surviving to the next year and produce the same number of
offspring.
^{11} *Linear*--The population will either grow geometrically or decline geometrically- Mathematical properties
- All age classes eventually grow (or shrink) at the same rate
- Initial growth depends on the age structure of the population
- Early reproduction contributes much more to population growth
rate than late reproduction.
In humans a woman who has three children starting at age 15 contributes as much to population growth as one who has five children starting at age 30.

- All age classes eventually grow (or shrink) at the same rate