Position Paper, Workshop on Evolutionary Computation with Variable Size Representation, ICGA, 20 July 1997, East Lansing, Michigan, USA

Fitness Causes Bloat

W. B. Langdon
School of Computer Science,
University of Birmingham, UK

• Bloat is inherent in discrete Var size + simple static unbiased fitness, not just GP

1. Search starts with short trial progs  
2. Rate of improvement quickly falls  
3. Fitness selects best in population  
4. 2. & 3. Fitness selects children which behave like their parents  
5. There are many more long programs than short which behave the same  
6. 1. & 4. & 5. Selecting one at random is likely to be longer than its parent
   I.e. Fitness causes bloat