Why engel curve slopes negatively downward in case of giffen goods?

Answer 1

The Engel curve shows how household expenditure on goods changes with rising income.

Giffen goods are inferior goods. As household income rises, instead of consuming more of the Giffen goods, expenditure is switched to better quality goods. Consequently, the demand for a Giffen good falls as income rises and this results in a downward sloping curve.

Incidentally, a curve that slopes "negatively downward" is actually a curve that slopes positively upwards!