It is well known that we get better at the things we practice, but is it possible to stimulate the brain with activities in such a way that general or specific intelligence is weakened? Also, are there skills that can be practiced that have been proven to weaken other skills?
While not necessarily lowering of one's IQ, two phenomena potentially detrimental to one's capacity for future learning and problem solving are Einstellung Effect and Compartmentalisation.
In the Einstellung Effect, an initial framing or initial experience with a particular type of problem or situation sets into habit a fixed way of behaving. Later, when a situation arises which evokes the same mental classification due to having a similar appearance, the person is left pursuing a suboptimal or perhaps dead-end approach. In other words, the person knows enough to be dangerous but may lack the insight needed to see this deficiency.
If one goes too long without encountering sufficient novelty, one's mind may become fixed. Neurologically this occurs because the same simple path is traversed too many times, very much like in addiction. Perhaps one solution is to move forward with one's learning and skills, never staying too long on the same level, always taking on new challenges, even when initial failure is likely. Staying with what is known may be comfortable, but staying too long closes the mind.
In Compartmentalisation, thoughts which conflict on some level of abstraction are separated into mutually exclusive mental boxes. A common example is separating work life from family life. If one partakes too readily in compartmentalising, however, the mind may become fragmented. The drive for compartmentalising is to escape cognitive dissonance, which, if done in excess, can slowly erode one's mental stability while dividing one's worldview into two or more inherently contradicting subsets. With standing contradictions, one's broken foundations may prevent the successful pursuit of higher truths.
Though not always practical, one solution to excessive compartmentalisation is to re-arrange one's life so as no longer to feel compelled to keep things separate. For example, one might find a career in better alignment with one's values. Another solution is to break down the mental barriers to integration by finding a more truthful, more encompassing worldview. One's existing perspective may have fallen prey to the Einstellung effect, having been set early in life without enough experience to see the holes.