Final answer:
In the 'Building Better Brains' article, fluid intelligence is tested by assessing the ability to tackle abstract challenges and novel problem-solving, which is part of Cattell's theory of intelligence.
Step-by-step explanation:
The article 'Building Better Brains' likely discusses fluid intelligence in the context of its definition by Raymond Cattell. Cattell characterized fluid intelligence as the ability to see complex relationships and solve problems, distinguishing it from crystallized intelligence which is based on acquired knowledge and the ability to retrieve it.
In relation to socioeconomic factors, research such as the one conducted by Kishiyama and colleagues has shown that external factors such as poverty can affect brain function and development, potentially impacting fluid intelligence and resulting in variations in IQ scores across different social groups.