Intellectual giftedness is an intellectual ability significantly higher than average. Giftedness is a trait that starts at birth and continues throughout the life-span. Giftedness is not a marker of success, but rather of aptitude or the inherent ability to learn. Gifted children often develop asynchronously; their minds are often ahead of their physical growth, and specific cognitive and emotional functions are often at different stages of development. Gifted individuals also experience the world differently, resulting in unique social and emotional issues. Some research suggests that gifted children have greater psychomotor, sensual, imaginative, intellectual, and emotional "overexcitabilities". Many schools use a variety of measures of students' capability and potential when identifying gifted children. These may include portfolios of student work, classroom observations, achievement measures, and intelligence scores. Most educational professionals accept that no single measure can be used in isolation to accurately identify a gifted child. Most IQ tests do not have the capacity to discriminate accurately at higher IQ levels, and are perhaps only effective at determining whether a student is gifted rather than distinguishing among levels of giftedness. Although the Wechsler tests have a ceiling of about 160, their creator has admitted that they are intended to be used within the average range (between 70 and 130), and are not intended for use at the extreme ends of the population.