Fluid intelligence is the ability to identify regularities in a problem met for the first time and process them logically, without depending on accumulated knowledge or experience. It does the central work in tasks such as inferring what comes next in a row of shapes or making sense of a structure never seen before. It forms a pair with crystallized intelligence, and the two differ in their trajectory as well: fluid intelligence is generally held to peak relatively early in adulthood and then decline gradually, whereas crystallized intelligence can keep growing across a lifetime.
Where It Does the Work
Solving a series reasoning item involves repeating several operations in turn. You break a figure into its attributes and find which of them is changing, form a hypothesis about how it changes, and test that hypothesis against the next element. If the hypothesis fails, attention shifts to a different attribute. Throughout this the hypotheses you have formed must be held in mind, which is why working memory capacity bears on performance. One explanation for why items in which several rules operate at once are hard is that the number of hypotheses to hold grows and presses against that capacity. The correlation between performance on visual reasoning and on working memory tasks is thought to arise from this shared foundation.
Measurement and the Influence of Culture
In measuring fluid intelligence, problems built out of words turn differences in native language and schooling into differences in score, so visual tasks dealing with changes in attributes such as form, count, orientation, and fill have been used instead. No task is entirely free of culture, but the influence of background is smaller than it is for a vocabulary item. Raven's Progressive Matrices, widely used as an index of non-verbal reasoning, is one such visual task.
Reading Changes That Come from Training
Repeat a task in the same format and the score will rise. This is likely because the procedure for breaking a figure into attributes becomes second nature and frequently recurring rules get committed to memory. A review of the peer-reviewed intervention studies cited on the websites of leading brain-training companies found extensive evidence that training improves performance on the trained tasks, but little evidence that it enhances performance on distantly related tasks.