Why do nuclei tend to have more neutron then protons at high mass numbers?

As the number of protons in a nucleus increases, it takes more neutrons to keep the nucleus stable - these "extra" neutrons contribute to the strong nuclear force and moderate the repulsive forces of the protons. See: Neutron Excess for Stable Nuclei (http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/neutexc.html) and The ABCs of Nuclear Research at HRIBF (http://www.phy.ornl.gov/hribf/science/abc/)