Internal causes of extinction are to be found in exaggeration of body size, in the hypertrophy or over-specialization of certain organs, in the irreversibility of evolution, and possibly, although this has not been demonstrated, in a progressive reduction of variability.
This is often read aloud and the irreversibility of the contract is symbolized by the breaking of a plate.
Gender constancy has been defined as children's understanding of the irreversibility of their sex, which develops in stages between about the ages of two and seven years.