According to Hicks's theory a change in factor prices will stimulate development of technical innovation to reduce the use of the factor for which the price increased compared to other factors. Following the oil shock significant investments were made in alternative sources of energy, more efficient cars and heating systems to mitigate oil consumption. As a result, emissions started growing at a slower rate than GDP per capita. Although petrol prices then dropped in the 1980s, emissions have continued to grow more slowly than GDP. This is an indication of a complete structural change in technology induced by the need to innovate.