In Old Testament times the Israelites were preparing for a war with the Midianites. Gideon was general of the Israelite army, and he had an unusual problem – too many volunteers. He needed to reduce their numbers, but not at random; he wanted to keep experienced and courageous soldiers. Gideon used two rough and ready tests. He first reminded his volunteers how great was their risk of death and injury: ‘Whosoever is fearful and afraid, let him return and depart early.’ About two-thirds of the volunteers changed their minds and went home. But ‘the Lord said unto Gideon, The People are yet too many’. Gideon told the remaining volunteers to drink from the nearest stream. Those that lapped the water like a dog ‘putting their hand to their mouth’ passed the test, but ‘all the rest of the people bowed down upon their knees to drink water’, and failed, because experienced soldiers keep watch for enemies at all times.

Gideon used first a personality test, then an aptitude test. Neither was very good by psychologists’ standards. Both are single item tests, so unlikely to give very reliable results. Gideon used one question, Are you afraid of fighting a battle? and one observation of fieldcraft (keeping watch while drinking). Gideon's personality test is the first recorded measure of what psychologists now call neuroticism or trait anxiety. The military have remained intermittently aware of individual differences and the need to assess them ever since. (Only intermittently: Cattell (1937) complained that the Army in 1914–18 ‘used some of the best brains from civilian life to stop bullets in front-line trenches’.) Modern methods of military selection are much more sophisticated than Gideon's two tests, but preserve the same distinction between personality (fearfulness) and ability (fieldcraft). This book deals only with personality; for a good review of ability, see McIntosh (1998), and of psychological testing, see Murphy and Davidshoffer (2005).