Again, again, we are here, discussing copyright extensions. Simple question: Why should society create an artificial monopoly on intellectual property? To reward people doing this work. How high does this reward need to be? High enough for people to be willing to do this work, to be motivated to do the work.
Let us see how much people get from a limited copyright. We compare the revenue stream from the copyright to the money they would get if they invested it at a certain rate. Now an invention will normally be most popular when conceived, and become less valuable over time. To model this, we simply add the rate of decline in popularity to our investment rate. So we treat 5% investment rate and 5% annual decline in revenues as a 10% alternative investment rate. This will give us the following table to see how much of the overall revenues you get from an invention or a copyrighted piece:
| Interest Rate | |||||||||||
| 1% | 2% | 3% | 4% | 5% | 6% | 7% | 8% | 9% | 10% | ||
| Duration in years |
10 | 9.5% | 18.0% | 25.6% | 32.4% | 38.6% | 44.2% | 49.2% | 53.7% | 57.8% | 61.4% |
| 20 | 18.0% | 32.7% | 44.6% | 54.4% | 62.3% | 68.8% | 74.2% | 78.5% | 82.2% | 85.1% | |
| 30 | 25.8% | 44.8% | 58.8% | 69.2% | 76.9% | 82.6% | 86.9% | 90.1% | 92.5% | 94.3% | |
| 40 | 32.8% | 54.7% | 69.3% | 79.2% | 85.8% | 90.3% | 93.3% | 95.4% | 96.8% | 97.8% | |
| 50 | 39.2% | 62.8% | 77.2% | 85.9% | 91.3% | 94.6% | 96.6% | 97.9% | 98.7% | 99.1% | |
| 60 | 45.0% | 69.5% | 83.0% | 90.5% | 94.6% | 97.0% | 98.3% | 99.0% | 99.4% | 99.7% | |
Table: Current value of a limited revenue stream compared to CV of the perpetual stream
It is quite interesting to see how quickly compound interest is reducing the value of an invention. I believe that 75% of the value to the inventor / creator and 25% to the public is more than a fair split to encourage innovation, and with a 5% depreciation rate which I believe to be pretty generous (many companies do not start projects that promise at least a 10% return), that would mean that 30 years are the correct duration of a copyright.