When it comes to the increasingly important role of test developer, hiring managers have a choice to make. They must decide what is the controlling criteria for hiring. Which is more important, testing skills or development skills? Sure, you want both to be strong but often times, you don't get a perfect candidate and are forced to compromise. If you have to choose one skill as the more important, which should it be? There is no universal answer. Each situation will vary. However, it is my assessment that you are often better off hiring a good developer and teaching testing skills rather than hiring a good tester and trying to teach development skills. The reason for this is largely this one belief: it is easier for the average developer to learn to test than for the average tester to learn to develop.
When I say developer, I don't just mean someone who knows the syntax of a programming language but rather someone who really understands programming. That is, someone with computer science competencies. This is not necessarily someone who has a CS degree. The person can be self-taught, but they need to understand the sorts of things computer science students learn. They need to have familiarity with data structures and algorithms. They must understand complexity issues. They should be familiar with operating system concepts including concurrency. For a modern programmer, experience with object-oriented design principles is also necessary.
Two critical traits all testers must have to pay attention to details and being curious. A tester needs to be thorough. They cannot afford to leave any stones unturned in their quest to explore the product. Good testers also need to be curious. They need to have an instinct where to look to find the issues. They have to want to experiment.
Let us think about most developers. Are they detail-oriented people? The good ones are. Code is unforgiving. If you don't pay attention to details, your program will misbehave or even crash. Are they curious? More often than not, curiosity is what brings someone into programming. Is it the sense of exploration and later mastery that drives most of us forward. It would seem then that good developers have the basic traits to be good testers.
What about the average tester? Here I'm not talking test developer but rather someone who is less skilled in programming. Do they have what it takes to make a good developer? There is no way of knowing. While curiosity and thoroughness are traits most developers have, those are not sufficient traits to be a good programmer. To be the best programmer one must be a good problem solver. One must also be able to think in very abstract ways. Most good testers are also good problem solvers. However, I'm not convinced that there is a correlation between abstract thinking and testing. Testing can be done in a very concrete manner. Many who make good testers do not have the ability to become great programmers. It is often this trait that is missing.
Now let us consider the specific skillsets required for testing and programming. It is argued that testing is just as difficult as programming. That the skillset required is equally deep and varied in manner rather than scope. I reject this notion.
It's not that I think testing is easy. It isn't. I've interviewed many people who don't have what it takes to be a good tester. They have the wrong mindset. They aren't curious or thorough enough. Sometimes they just don't really grok computers. Often times the higher level skills in testing are enumerated to prove the difficulty. These include things like understanding equivalency classes or pairwise testing. The thing is, I've never found these to be too difficult to understand. Equivalency analysis--despite its fancy name--is something any competent tester should just intuitively understand. Pairwise testing is more complex but is something any CS student should be able to understand quickly. There is a lot of art in successful testing too. This can take a long time to develop well. The average developer isn't going to be a great tester overnight.
How about programming? How easy is it for the average tester to pick it up? As I argued above, good programming is not just knowing the syntax. I like to use the concept of writing to prove my point. Knowing the syntax of a programming language is like knowing grammar. You can correctly form sentences and can most likely convey your point to the audience. However, knowing grammar does not make you a good writer. What sets Mark Twain apart from your typical math student is not just a superior knowledge of grammar and vocabulary. It is understanding the larger process at work. Likewise, knowing the syntax of C++ or Java doesn't make for a good programmer. One merely needs to watch The Daily WTF for a short while to understand why such thinking can cause enormous trouble over time. It takes a long time and a lot of hard work to go from being someone who understands syntax to someone who can truly weave code.
Thus I think the gap between a tester and a developer is harder to cross from the tester side than from the developer side. A developer can become an adequate tester well before a tester can become an adequate developer. Given the choice between someone who is a good developer and someone who is a good tester and each lacking much skill in the other discipline, I'll take the developer nearly every time.
As always, your mileage may vary. There are some testers who go on to make amazing developers. There are developers who cannot grok testing. There are some jobs where you don't really need good development skills. Sometimes a person who understand syntax is enough.
That's my take anyway. Release the arrows.
A few related articles are Chris McMahon's discussion of developer-testers vs tester-developers. Also Elisabeth Hendrickson has a great post about the convergence of testing and development.