“Data expands to fill the space available for storage”; buying more memory encourages the use of more memory-intensive techniques. (The original 1958 Parkinson's Law described the structural tendency of bureaucracies to make work for themselves.) It has been observed since the mid-1980s that the memory usage of evolving systems tends to double roughly once every 18 months. Fortunately, memory density available for constant dollars also tends to about double once every 18 months (see Moore's Law); unfortunately, the laws of physics guarantee that the latter cannot continue indefinitely.