Poster presentation for Late-Breaking Papers at the Genetic Programming 1997 Conference, J. R. Koza (ed)

Fitness Causes Bloat

W. B. Langdon

Computer Science, The University of Birmingham, B15 2TT, UK

W.B.Langdon@cs.bham.ac.uk http://www.cs.bham.ac.uk/wbl

Tel: +44 (0) 121 414 4791, Fax: +44 (0) 121 414 4281

Fitness Causes Bloat

1. GP starts with short trial programs  
2. Rate of finding improvements 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

• Example -- The Artificial Ant Problem
• Tree Mutation
• Acknowledgements
• About this document ...