Most Open Source programmers would probably not write software and give it away if it cost them something to do so. However, they don't perceive the effort of writing it to be a cost. Most of them write software to solve a specific problem that they happen to be facing, or to "scratch their personal itch", as Eric Raymond points out. The process of developing such software is actually quite pleasurable and energizing to most good programmers. Once the software has been written, giving away copies of it does not deprive the programmer of the ability to continue to use it, and it costs them nothing extra to do so. It is a win-neutral transaction, and therefore there is no economic reason to prevent it from taking place.

Economics purists would point out that there is indeed a cost to giving away the software--the opportunity cost of not selling the software instead. However, for many programmers, the process of selling their software is more trouble than it is worth, so the effective opportunity cost is actually zero, and it is a win-neutral situation after all.

If that was not sufficient reason, Open Source programmers also tend to work with others who share their interest and contribute code. They enjoy a multiplier effect from such cooperation. Metaphorically speaking, each programmer contributes a brick and each gets back a complete house in return. In software, unlike with physical goods, one person's gain does not come at the expense of another because a copy does not deplete the original in any way. Sharing software is not a zero-sum game, and there are tremendous efficiencies from participating in such a cooperative endeavor.

No, the absence of direct monetary reward does not really constitute a disincentive to writing Open Source software.

"Even Open Source development involves effort, so there has to be payment for that effort"

OK, Open Source programmers lose nothing by giving away the software that they have already written (and they may even gain in non-monetary terms). But some effort has gone into their products. Shouldn't such effort be compensated in cash as well? Programmers have families to support, and they need to put bread on the table. They can't live on software and satisfaction alone. To be viable in the long term, Open Source needs to evolve a mechanism to support its contributors financially. Without remuneration, over time, most of these volunteer programmers will simply wander away in search of food.