Why Is the Slope of the Indifference Curve Always Negative?

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SUMMARY

The slope of the indifference curve is always negative due to the definition of the marginal rate of substitution (MRS), which is represented as MRS = -(dy/dx). This negative slope indicates that as the quantity of good X increases, the quantity of good Y must decrease to maintain constant utility. A positive slope would imply that increasing one good would also increase the other, contradicting the principle of constant utility along the curve.

PREREQUISITES
  • Understanding of marginal rate of substitution (MRS)
  • Familiarity with indifference curves in consumer theory
  • Basic knowledge of utility theory
  • Graphical interpretation of slopes in economics
NEXT STEPS
  • Study the implications of the marginal rate of substitution in consumer choice theory
  • Explore the properties of indifference curves and their shapes
  • Learn about the concept of utility maximization
  • Investigate the relationship between MRS and budget constraints
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Economics students, educators, and anyone interested in understanding consumer behavior and utility theory.

garyljc
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I understood the concept and definition of marginal rate of substitution , and also
MRS = -(dy/dx) , but why is dy/dx never positive ?
 
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The simplest explanation is to remember that MRS is the slope of an indifference curve. Your question is the same as "why does an indifference curve always slope downwards"? Think about the axes of the graph on which an indif. curve is plotted. What does a negative slope mean in terms of how y changes when you increase the amount of x? (What would it mean in terms of how x changes when you increase the amount of y?) How would your answers be different if the slope was positive? You should keep in mind that utility has to be constant on each indifference curve.
 

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