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Mechanical aptitude is a nonverbal ability.
According to Paul Muchinsky in his textbook Psychology Applied to Work, "Mechanical aptitude tests require a person to recognize which mechanical principle is suggested by a test item. The underlying concepts measured by these items include sounds and heat conduction, velocity, gravity, and force. One of the more popular tests of mechanical reasoning is the Bennett Test of Mechanical Comprehension. The test is a series of pictures that illustrate various mechanical concepts and principles. Other tests of mechanical comprehension have also been developed. Tests of mechanical ability are highly predictive of performance in manufacturing/production jobs. However, women traditionally perform worse than men on tests of mechanical ability. Recent attempts to include test questions pertaining to kitchen implements and other topics about which women are more familiar have reduced, but not eliminated, the male/female score differential." 
Aptitude tests have been used for military purposes since World War I to screen recruits for military service. The Army Alpha and Army Beta tests were developed in 1917-1918 so ability of personnel could be measured by commanders. The Army Alpha was a test that assessed verbal ability, numerical ability, ability to follow directions, and general knowledge of specific information. The Army Beta was its non-verbal counterpart used to evaluate the aptitude of illiterate, unschooled, or non-English speaking draftees or volunteers.
During World War II, the Army Alpha and Beta tests were replaced by The Army General Classification Test (AGCT) and Navy General Classification Test (NGCT). The AGCT was described as a test of general learning ability, and was used by the Army and Marine Corps to assign recruits to military jobs. About 12 million recruits were tested using the AGCT during World War II. The NGCT was used by the Navy to assign recruits to military jobs. About 3 million sailors were tested using the NGCT during World War II.
Additional classification tests were developed early in World War II to supplement the AGCT and the NGCT. These included:
- Specialized aptitude tests related to the technical fields (mechanical, electrical, and later, electronics)
- Clerical and administrative tests, radio code operational tests
- Language tests and driver selection tests.
Mechanical Aptitude and Spatial Relations
Mechanical aptitude tests are often coupled together with spatial relations tests. Mechanical aptitude is a complex function and is the sum of several different capacities, one of which is the ability to perceive spacial relations. Some research has shown that spacial ability is, for some jobs, the most vital part of mechanical aptitude. Because of this, spacial relations tests are often given separately or in part with mechanical aptitude tests. 
There is no evidence that states there is a general intelligence difference between men and women. However, there is a difference in the cognitive abilities between the two sexes. Males tend to do better on the quantitative tasks and the visual-spatial skills then females, and females tend to score much higher than men on the verbal sections of the test. Males have always performed very high on the mechanical aptitude tests; much higher than women. In recent years, another mechanical aptitude test was created. The main purpose of this test was to create a fair chance for women to perform higher than, or at the same level as men. Males still perform at a much higher level than women, but the scores between men and women have been drawn closer together. There is little research that has been devoted as to why men are able to complete the tests and perform much higher than women.
Mechanical aptitude measurement
- Main article: Mechanical aptitude measurement
A number of test have been developed to assess mechanical aptitude.
- Muchinsky, Paul (2012). Psychology Applied to Work, 105, Summerfield, NC: Hypergraphic Press.
- History of Military Testing. URL accessed on 25 April 2012.
- Levy, Joan (2004). Master the Mechanical Aptitude and Spatial Relations Tests, 2, Thomson Peterson's.
- Christensson Ringby, Karolina (2001-01-01). Sex Differences in Mechanical Aptitude: An Investigation of Sex Differences in Mechanical Aptitude and Its Relation to Nonverbal Abilities. the Osprey Journal of Ideas and Inquiry All Volumes: 53–57.