Another full treatment is that of Titchener, in his Textbook of Psychology, 1909, pp. 265-302.
On the topic of distraction, see John J. B. Morgan's Overcoming of Distraction and Other Resistances, 1916.
CHAPTER XII
INTELLIGENCE
HOW INTELLIGENCE IS MEASURED, WHAT IT CONSISTS IN AND EVIDENCE OF ITS BEING LARGELY A MATTER OF HEREDITY
Before leaving the general topic of native traits and passing to the process of learning or acquiring traits, we need to complete our picture of the native mental constitution by adding intelligence to reflex action, instinct, emotion, feeling, sensation and attention. Man is an intelligent animal by nature. The fact that he is the most intelligent of animals is due to his native constitution, as the fact that, among the lower animals, some species are more intelligent than others is due to the native constitution of each species. A rat has more intelligence than a frog, a dog than a rat, a monkey than a dog, and a man than a monkey, because of their native constitutions as members of their respective species.
But the different individuals belonging to the same species are not all equal in intelligence, any more than in size or strength or vitality. Some dogs are more intelligent than others, and the same is notably true of men. Now, are these differences between members of the same species due to heredity or environment? This question we can better approach after considering the methods by which psychologists undertake to measure intelligence; and an analysis of these methods may also serve to indicate what is included under the term "intelligence".
Intelligence Tests
Not far from the year 1900 the school authorities of the city of Paris, desiring to know whether the backwardness of many children in school resulted from inattention, mischievousness and similar difficulties of a moral nature, or from genuine inability to learn, put the problem into the hands of Alfred Binet, a leading psychologist of the day; and within a few years thereafter he and a collaborator brought out the now famous Binet-Simon tests for intelligence. In devising these tests, Binet's plan was to leave school knowledge to one side, and look for information and skill picked up by the child from his elders and playmates in the ordinary experience of life. Further, Binet wisely decided not to seek for any single test for so broad a matter as intelligence, but rather to employ many brief tests and give the child plenty of chances to demonstrate what he had learned and what he could do. These little tests were graded in difficulty from the level of the three-year-old to that of the twelve-year-old, and the general plan was to determine how far up the scale the child could successfully pass the tests.