The interaction between Engel's law, technological progress, and the process of structural change is crucial for explaining long-term economic growth as suggested by Leon, and Pasinetti. Engel curve A concept which is based on Engel's law is Engel curve. This curve shows how the spending on a certain good varies by either the proportion or absolute dollar amount. The shape of the curve depends on factors such as age, gender, educational level and which type of good it is visualising. Moving along the curve assumes that these factors are held constant, ceteris paribus, while only income is changed. Logically, when demographic factors change, the Engel curve shifts accordingly. Moreover, Pope (2012) notes that larger families will have higher food consumption, as well as that consumption expenditures change throughout a life cycle, even holding income and other demographic variables are held constant. The share of a budget spent on food rise and then fall during a lifetime, producing an inverted parabola, indicating that expenditures on food as well as total expenditures peak during mid-life. With income level as the x-axis and expenditures as the y-axis, the Engel curves show upward slopes for normal goods, which have a positive income elasticity of demand. Inferior goods with negative income elasticity, assume negative slopes for their Engel curves. In the case of food, the Engel curve is concave downward with a positive but decreasing slope. Engel argues that food is a normal good, yet the share of household's budget spent on food falls as income increases, making food a necessity. History of Engel's law In 1857, by using data from several households, Ernst Engel found that household food expenditure and household income are correlating. The relationship between food share of household expenditure and household income was also highly consistent. The income elasticity for food expenditure was inelastic, below 1 (around .86) for each gathered dataset, meaning it is insensitive to price changes. By 1875 Engel's law had spread across the Atlantic to Europe. There it gained attention and was used by Carroll D. Wright, which did not only argue that food expenditure was the only thing that varied with income, but that in fact clothing, housing and sundries (meaning expenditures that are not frequently incurred, like home improvement items) also varied with income in agreement with Engel’s law. In 1932 Zimmerman stated that Engel's law had been utilised for other factors than income to