There are three reasons why YAGNI works so well. First, people systematically underestimate uncertainty. The human brain is good at recognizing patterns and ma

There are three reasons why YAGNI works so well. First, people systematically underestimate uncertainty. The human brain is good at recognizing patterns and making plans but not so good at dealing with randomness and uncertainty. When talking about software development and maintenance we're talking timescales of years. Companies go out of business, projects get cancelled, code gets thrown out and rewritten. It's easy to paint a pretty picture about how the Cat class will get added to the Pet hierarchy but there is tons of uncertainty involved in even the simplest project. Because the scenario is specific the brain overvalues its likelihood compared to the nebulous unknown alternatives. Second, keeping code as simple as possible is almost always for the best. YAGNI forces smaller simpler code. More complexity and more code means more room for bugs. All the studies I know of consistently show that more code means more bugs. Finally YAGNI is a principle that forces priorities to be in the right place. The question is never between spending time making something that might be useful in the future or doing nothing, it's between doing one particular useful thing or another. I can't think of a single case where the right priority is to do something that MIGHT help in the future. You start by doing things that DO help now, then things that WILL help in the future.