The Broken Windows Theory
The Broken Windows Theory on Wikipedia
The Broken Windows Theory suggests that visible signs of crime (or lack of care of an environment) lead to further and more serious crimes (or further deterioration of the environment).
This theory has been applied to software development, suggesting that poor quality code (or Technical Debt) can lead to a perception that efforts to improve quality may be ignored or undervalued, thus leading to further poor quality code. This effect cascades leading to a great decrease in quality over time.