When an Admiral reaches his retirement age, he will begin to consider the possibility of leaving military service. The chance that the Admiral chooses retirement, increases with every passing turn past the minimum age of retirement, as he starts to feel the effects of old age.
If the Admiral chooses to retire, he will in some cases attempt to train his successor to take command of the fleet, and there is the chance that he will succeed in passing a Special Characteristic to his successor.
This chance is dependent on each race, all races except the Hiver have a 20% chance and the Hivers a 50% chance of success due to their unique physiology - see the game encyclopedia for "Reincarnation".
With special research on the Psionic branch of the tech tree, the player can increase the odds in his favor.