If one concept from cognitive psychology is applied to product development what should it be?

If you're serious about creating a good product and experience, you can't pick just one. Luckily, much has been written about how psychology can inform design. Don Norman (http://www.jnd.org/) would be a great place to start. He's not only the father of User Centered Design, but he also made significant contributions to Cognitive Psychology (particularly his work on Executive Function) before focusing on product design and experience.

Flow [1]. The state of flow should be the natural outcome of good design for the products used for extended period of time ("extended" is variable, depends on person and useage). Design of flow disruption is equally important for error prevention and recovery. [1] http://en.wikipedia.org/wiki/Flow_%28psychology%29

If you were building a product that could possibly come in different variations, then you might consider Barry Schwartz's Paradox of Choice that states that it is more difficult to choose between many things, and easier to choose when there is only a few. I'd argue that for something to be successful it almost needs to be a single compelling thing with maybe some minor variations. Making a 'range' is harder work than making one brilliant thing, so it makes production sense too. Lets face it, you don't get 10 different sizes of, say, Phillipe Starcks Lemon Squeezer! Perhaps Apple considered this when designing the iPhone ...

I have put this answer in another question - if I should link to it or if I need to delete this answer as it is a duplicate - please let me know Po - Edward Debono http://en.wikipedia.org/wiki/Po_%28lateral_thinking%29 A  precondition for an idea that means neither yes or no, but "let's examine this openly and freely and only make decisions after we have "po"ed it. A short hand way of getting agreement to not judge the question before we have sufficient information or sufficient discussion on the possible answers.  This word would be a great addition to the Quora lexicon, if the meaning becomes generally known

I would say priming effects. Priming is one of the most researched things in psychology. It says that exposure to a stimulus influences your response to a later stimulus. Which is why first-sight design is so important. If something elicits a feeling of happiness or wonder then it is priming the person for when the functionality interacted with. Here are possible research questions: What elements in the physical world elicit positive responses based on sight? What elements in products elicit positive responses based on sight?

If you really wanted to apply just one, go for  the thinking round default options - our tendency to go with whatever default option is selected for us. Interesting, for example, how Quora offers a clear Logout option in the header, whereas on Facebook (which never wants you to logout) the option to logout is harder to find (ie. not so default, if that makes sense).

loops. time spent on site = money. doesnt matter what the product is if the experience is authentic and engaging. memory of experience linked to product leads to purchases and consumer returns i.e. - discontinuing links to facebook, twitter, and pitnerest on every single page of your site would help to start