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Subject BUSINESS ECONOMICS

Paper No and Title 1: Microeconomics Analysis

Module No and Title 6: Indifference Curves

Module Tag BSE_P1_M6

BUSINESS PAPER NO.1 : MICROECONOMICS ANALYSIS

ECONOMICS MODULE NO.6 : INDIFFERENCE CURVES
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TABLE OF CONTENTS
1. Learning Outcomes
2. Introduction
3. Consumer Preferences
4. Indifference Curves and Indifference Map
5. Marginal Rate of Substitution
5.1 Principle of Diminishing MRS
5.2 Marginal Rate of Substitution and Marginal Utility
6. Properties of Indifference Curves
7. Exceptional Shapes of Indifference Curves
7.1 Perfect Substitutes and Perfect Complements
7.2 Good, Bad and Neutral commodities
8. Summary

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1. Learning Outcomes
After studying this module, you shall be able to learn about

 Meaning of consumer preferences

 Concept of indifference curve and indifference map
 The Law of Diminishing Marginal Rate of Substitution
 Relationship between Marginal Rate of Substitution and Marginal Utility
 Properties of Indifference curves
 Perfect Substitutes and Perfect Complements
 Good, Bad and Neutral commodities

2. Introduction
Indifference curve analysis is a very popular method used to explain consumer behavior. The
technique of indifference curve was first invented by Edgeworth (1881) and then by Fischer (1892).
Later on the Italian economist Pareto (1906) put it to extensive use and the results were
subsequently extended by Soviet economist Slutsky (1915).

We have already explained the concepts of cardinal and ordinal utility theories. Modern
indifference curve approach uses the concept of ordinal utility. The indifference curve analysis
assumes that the consumer have complete information about all the aspects of economic
environment. Further, the consumer is assumed to act rationally, so given the money income and
prices of goods, he will choose the combination from among various alternatives that gives him
maximum satisfaction.

3. Consumer Preferences
Preference means choosing one alternative over others. To analyze the preferences under two
dimensional set up, the commodities, which a consumer can buy, can be divided into two groups
as good X and good Y. The consumer’s choice among various alternatives depends upon his
preferences, that is, the ranking he gives to various alternatives. Other factors, for example income
and prices also influence his choice of an alternative. In indifference curve approach of consumer
behavior, certain important assumptions about the nature of consumer’s preference are the
following.
1. Completeness: Under this assumption, the consumer is capable of comparing all the
alternative combinations and ranks them according to utility. Given two bundles P and Q,
he can decide whether he prefers P to Q, Q to P or he is indifferent between the two.
2. Transitivity: Transitivity of preference means that if a person prefers a combination P to
Q and also prefers Q to R, then he will prefer P to R. Thus transitivity implies that
consumer’s taste and preferences are consistent.

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3. Non–Satiation: More of a good is always preferable to less of that good. Put differently,
“more is better”, other things remaining constant. The assumption implies that the
individual is not already over supplied or over satiated with any good and that the good is
desirable.
4. Indifference Curves

4. Indifference Curves
An indifference curve represents the various alternative combinations of two commodities, which
give the same level of satisfaction to the consumer – so the consumer is ‘indifferent’ between these
combinations. The indifference curve is a graphical representation of indifference schedule, where
all the alternative combination of commodities gives exactly the same satisfaction level or utility
to the consumer. To explain it further, consider the example below:

Table 1

Combination Unit of good ‘X’ Unit of good ‘Y’

P 1 100
Q 2 45
R 3 25
S 4 15
T 5 10

The given table shows that the consumer is indifferent between the given five alternative
combinations (P, Q, R, S and T) of two goods (X and Y). When all the combinations are represented
in the form of a graph, we obtain an indifference curve as shown in the figure 1.

[Figure 1: An indifference curve]

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Figure 1 shows the quantities of commodity ‘X’ on the horizontal axis and of commodity ‘Y’ on
the vertical axis. All of the five combinations (shown by points P, Q, R, S and T) fall on same
indifference curve indicating that these alternative combinations give equal utility to the consumer.
The indifference curve is also called iso-utility curve (“iso” means same) as it represents equal level
of satisfaction at every point on the curve.

Note, as we move down the indifference curve, an increase in quantity of good X is offset by a
decrease in quantity of good Y, so as to maintain the same level of satisfaction along the curve.

4.1 Indifference Map

One indifference curve shows consumption bundles providing single level of satisfaction. We can
now think of different indifference curves for each given satisfaction level. In fact, we can draw
any number of indifference curves representing different level of utility. An indifference map is the
whole set of indifference curves representing the consumer’s taste and preferences (figure 2).
Higher indifference curve showing greater quantities of both commodities represent higher
satisfaction levels as ‘more is better’. On the other hand lower indifference curves showing lesser
quantities represents a lower satisfaction level.

[Figure 2: Indifference map]

In the above diagram indifference curve shown as IC1 represents the lowest satisfaction level while
the curve IC4 represents the highest satisfaction level. Along any given indifference curve the level
of satisfaction remains same at all points.

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5. Marginal Rate of Substitution
Marginal rate of substitution (MRS) is one of the most basic concepts of the indifference curve
approach. It shows the rate at which a consumer is willing to exchange commodities X and Y along
a given indifference curve. More specifically, marginal rate of substitution of X for Y (MRSX, Y) is
the amount of good Y, which the consumer is willing to give up as he gets one additional unit of
good X, so that his satisfaction level remains the same. In our example (table-1), when a consumer
moves from P to Q, the consumer sacrifices 55 unit of ‘Y’ for one extra unit of ‘X’ and still remains
on the same indifference curve. Therefore, at this stage, MRSX,Y is equal to 55. The movement is
shown below in Figure 3.

[Figure 3: Marginal Rate of Substitution]

When a consumer moves from point P to Q on this indifference curve he sacrifices ΔY for ΔX and
remains at the same satisfaction level. This means the marginal rate of substitution of X for Y
(MRSX,Y) is ΔY/ ΔX (=PA/AQ).

MRSX,Y = (change in Y) / (Change in X)

Now, if the points P and Q are very close, the values of ΔY and ΔX will be very small. Then the
MRSX,Y will be given by the slope of the tangent to the indifference curve at that point (Figure 5).

5.1 Principle of Diminishing Marginal Rate of Substitution

In our example (table 1) we observe that, when the consumer moves from point P to T, he does not
have to sacrifice same amount of good Y for each extra unit of good X. Rather, he sacrifices lesser
and lesser amount of commodity Y for each extra unit of X as he moves towards point T. This
behavior of the consumer is said to be satisfying the principle of diminishing marginal rate of
substitution which says that the MRSX,Y diminishes as more and more good X is substituted for

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good Y. The principle of diminishing marginal rate of substitution is illustrated diagrammatically
in figure 4a and figure 4b.

[Figure 4a and figure 4b: Diminishing Marginal Rate of Substitution]

It is evident from the figure 4a that, as the consumer move downwards along the IC, the length of
ΔY becomes shorter and shorter while ΔX is kept the same. Put differently, the MRSX,Y falls as the
consumer gets more of good X and less of good Y. The diminishing MRSX,Y can also be
demonstrated by drawing tangents at different points on the same indifference curve. The MRSX,Y
at any point is given by the slope of the tangent at that point on the indifference curve. It is clear
from the Figure 4b that the slope of the tangent declines as we go down the indifference curve.
From the law of diminishing marginal utility we know that, as consumption of X increases, its
marginal utility decreases. Therefore, the consumer sacrifices lesser amount of good Y for each
additional unit of good X. Thus MRSX,Y falls when the amount of X is increased.

5.2 Marginal Rate of Substitution and Marginal Utility

It can be shown mathematically that MRSX,Y between two commodities is equal to the ratio of
marginal utilities of commodities X and Y.

Since utility on each point of the indifference curve remains the same we can represent an
indifference curve by

𝑈(𝑥, 𝑦) = 𝑎 --------------------------------- (i)

Where ‘a’ represent the constant utility along indifference curve and x and y are the amount of
commodities X and Y respectively. Taking total differentiation of (i), we get

𝜕𝑈 𝜕𝑈
𝑑𝑥 + 𝑑𝑦 = 0
𝜕𝑥 𝜕𝑦

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𝜕𝑈
𝑑𝑦 𝜕𝑥
− = 𝜕𝑈
𝑑𝑥
𝜕𝑦

∂U/∂x and ∂U/∂y are marginal utilities of goods X and Y respectively. Thus

𝑑𝑦 𝑀𝑈𝑥
− = = 𝑀𝑅𝑆𝑥, 𝑦
𝑑𝑥 𝑀𝑈𝑦

Thus the MRS between two goods is equal to the ratio between the marginal utility of two goods.

6. Properties of Indifference Curves

The main properties of indifference curves can be deduced from the assumptions about the nature
of preferences upon which indifference curve analysis is based.

(1) Indifference curves slope downward to the right: This means that indifference curves
are negatively sloped. This follows from the assumption of non-satiation of preferences,
that is, more of a good is always preferable to less of that good. Negatively sloped
indifference curve shows that an increase in the amount of a good must be accompanied
by the decrease in amount of the other, so that satisfaction level remains same.

Let us consider a change in combination from point A to point B in a way that the quantity
of good X increases, while Y remains same. This will make B more preferable in
comparison to the original combination A and therefore the combination A and B will not
be on the same indifference curve.
Instead, of being negatively sloped if indifference curves were horizontal, the consumer
would be indifferent between the above two combinations, point A and point B. But this
would violate our assumption of non-satiated preferences, ruling out horizontal
indifference curves when the two goods are normal. The situation is illustrated in figure
5a.

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[Figure 5a: Horizontal straight line indifference curve]

[Figure 5b: Vertical straight line indifference curve]

Likewise, with two normal goods an indifference curve cannot be represented by a vertical straight
line (figure 5b) as in this case, combinations A and B yield same satisfaction level having same
amount of X, but B contains more Y than A. Further, an upward or positively sloped indifference
curve (figure 5c) means that the combinations A and B yield equal satisfaction, though B contains
more of goods X and Y. But this cannot be so, if both the goods have positive marginal utilities.

[Figure 5c: Upward sloping indifference curve]

[Figure 5d: Normal indifference curve (downward sloping)]

In all the three situations discussed above, the level satisfaction of the consumer rises, as he moves
from point A to point B since he starts to consume more of at least one commodity. Therefore
normal indifference curves cannot be horizontal, vertical or upward sloping. Thus, as a last
possibility, an indifference curve must be downward sloping as shown in figure 5d. Negatively
sloped indifference curve indicates that, for a consumer to get same satisfaction level from various
combinations, as the quantity of good X is increased, the quantity of Y must be reduced.

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(2) Indifference curves are convex to the origin: indifference curves are usually convex to
the origin which means that the left portion of an indifference curve is relatively steeper
while the right portion is relatively flatter. Thus, with movement from left to right, down
the indifference curve the value of its slope (absolute) declines. This property of the
indifference curve follows from the principle of diminishing marginal rate of substitution
discussed in section 5.1. A convex shaped indifference satisfies the condition of
diminishing MRSX,Y as is clear from the figures 6a where, as the consumer moves down
along the IC from A to F, he gives up lesser and lesser amount of Y for each additional unit
of X. This is because, when the consumer reduces his consumption of good Y while
increasing his consumption of good X, the urge for more units of X declines continuously,
making the consumer sacrifice fewer Y for each extra unit of X. Instead, if the indifference
curve was concave this would imply that MRSX,Y increases as more and more units of X
are substituted for Y.

[Figure 6: Diminishing MRSXY on a convex indifference curve]

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[Figure 7: Increasing MRSXY on a concave indifference curve]

It is clear from figure 7 that, for each extra unit of good X, the consumer is ready to sacrifice
more and more quantity of Y, that is, MRSX,Y increases as more quantity of good X is
substituted for good Y. Thus, an indifference curve that is concave to the origin violates
the principle of diminishing marginal rate of substitution. Therefore, in the presence of
diminishing marginal rate of substitution, we cannot have concave indifference curves.

(3) Two indifference curves cannot intersect or touch each other: Intersection of two
indifference curves, representing two different levels of satisfaction, is not possible. This
property follows from the assumptions of transitivity and non-satiation of preferences. This
property can be proved through the method of contradiction. Suppose two indifference
curves intersect (or touch) each other at point P (figure 8 and 9). Now consider point A on
indifference curve IC1 and point B on IC2 which is vertically above point A. Consumer will
be indifferent between combinations A and P since both are lying on the indifference curve
IC1. Further, the combinations B and P will provide equal utility to the consumer as they
both lie on same indifference curve IC2. Applying transitivity assumption, it follows that
the combination A will be equivalent to B in terms of satisfaction. But combination B lies
on a higher indifference curve and contains more of good Y than combination A while the
amount of X is same in both the combinations. Thus, the consumer will definitely prefer B
to A if the assumption of non-satiation of preferences holds. But the intersection of two
indifference curves leads to a conclusion of A being equal to B in terms of utility or
satisfaction which is not possible. Therefore it can be concluded that the indifference
curves cannot intersect or touch each other.

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[Figure 8: Indifference curves cannot intersect]

[Figure 9: Indifference curves cannot touch]

(4) Higher indifference curves represent higher level of satisfaction: A higher indifference
curve will represent a higher satisfaction level than a lower indifference curve. Put
differently, the combinations lying on higher indifference curve will provide greater
satisfaction than the combinations lying on lower indifference curve. In the figure 10, IC2
is a higher indifference curve than IC1. Therefore, the combination B which lies on
indifference curve IC2 will provide the consumer more satisfaction than combination A
which lies on IC1. This is because the combination B contains more of both commodities
X and Y than the combinations A. Applying the assumption of non-satiety, the consumer
must prefer B to A. Now, using transitivity assumption it can be said that the consumer
will prefer any combinations on IC2 to any combination on IC1. We therefore conclude that
a higher indifference curve represents a higher level of satisfaction.

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[Figure 10: Higher indifference curve shows higher level of satisfaction]

7. Exceptional shapes of indifference curves

As discussed in the previous sections, standard indifference curves are convex as implied by the
principle of diminishing marginal rate of substitution. However, under certain circumstances, the
shape of indifference curves may not necessarily be strictly convex as discussed below.

7.1 Perfect Substitutes and Perfect Complements

Two commodities are said to be perfect substitute, if one good can be substituted for the other good
at a constant rate. Put differently, the consumer is ready to sacrifice same quantity of one
commodity for each additional unit of other commodity. This means that the marginal rate of
substitution between the two commodities is constant and thus the indifference curve is linear. Thus
a straight line indifference curve implies that MRSXY remains constant as more and more unit of X
substituted for Y. As explained in figure 11, on a straight line indifference curve consumer is willing
to give up same amount of Y for each additional unit of X (ΔY1 = ΔY2 = ΔY3 = ΔY4) as more and
more unit of X substituted for Y. Since MRSXY is equal to the slope of indifference curve at a point
on it, and because a straight line has same slope throughout, therefore the straight line indifference
curve will mean the same MRSXY throughout. However, in real life, goods tend to be imperfect
substitutes and therefore the indifference curves are not normally a straight line.

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[Figure 11: Perfect substitutes]

[Figure 12: Perfect complements]

Now, we consider the case of perfect complements, which are consumed together in a fixed
proportion. Perfect complements cannot at all be substituted for each other and are therefore
marginal rate of substitution between such goods is zero. A consumer needs fixed quantities of both
the goods (for example left and right shoe) to give him satisfaction. He cannot maintain satisfaction
by substituting one commodity for the other. In figure 12, the indifference curve for perfectly
complementary goods will be L-shaped or right angled, with the vertex of L showing the proportion
in which the two complementary commodities are used. Here the satisfaction level can only be

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increased by simultaneous increase in the quantity of the two commodities in the fixed proportion
which leads the consumer to a higher indifference curve.

7.2 Good, Bad and Neutral commodities

If a commodity is ‘good’, more of the commodity is always preferable to less of that commodity.
We have explained the indifference curves of such commodities which slope downward and are
convex to the origin. On the contrary, when a commodity is a ‘bad’, more of it will lower the
consumer’s satisfaction. Such ‘bads’ (for example, pollution) are undesirable and provide disutility
to the consumer. Therefore the indifference curves in this case assume a different shape. Suppose
a bad is represented on X-axis and a good is represented on Y-axis, then the indifference curve will
be positively sloped as shown in figure 13a. This is because, a movement towards the right along
the indifference curve reduces the consumer’s satisfaction level due to increased consumption of
‘bad’ and therefore, to keep his satisfaction level constant, the amount of ‘good’ has to be increased.
Thus the direction of preference in this case is upward to the left (Figure 13a). If commodity X is
a ‘good’ and commodity Y is a ‘bad’ the indifference curves are upward sloping to the right as
shown in figure 13b.

[Figure 13a: Indifference curves between ‘bad’ and ‘good’]

[Figure 13b: Indifference curves between ‘good’ and ‘bad’]

When commodity X and Y both are ‘bads’ indifference curves will be downward sloping and
concave to the origin as shown in figure 14. If the consumption of one bad is increased, the
consumption of other bad must fall so that the fall in satisfaction level due to increase in
consumption of one bad can be compensated by increase in the satisfaction level due to decrease
in consumption of other bad. As the consumer goes down the indifference curve absolute slope
increases and hence the MRSX,Y increases. This means that equal successive increase in the amount
of X will have to be compensated by more and more reduction in quantity of Y.

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[Figure 14: Indifference curves when both commodities are ‘bad’]

In the case of ‘neuters’ or ‘neutral’ goods the consumer does not care whether he has more or less
of that commodity. In other words, marginal utility of such commodity is zero and they do not
provide any satisfaction to the consumer. If a commodity X is a neuter and Y a normal commodity,
the indifference curves will be horizontal straight lines as shown in the figure 15a indicating that a
higher satisfaction level can only be achieved from increasing the consumption of the normal good.
On the other hand, if commodity Y is neuter and commodity X is a normal good, then the
indifference curves will be vertical straight lines as depicted in figure 15b. In this case, to increase
the satisfaction and therefore to reach a higher indifference curve, the consumer must acquire more
of normal good X as commodity Y does not provide any utility to the consumer.

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[Figure 15a: Indifference curves between a ‘neuter’ and ‘good’]
[Figure 15b: Indifference curves between ‘good’ and ‘neuter’]

A commodity may be a ‘good’ up to a point, called the point of satiation and then becomes ‘bad’
if the consumer is forced to increase his consumption beyond that point. In short, far too much of
any good may be bad. Let us suppose that the commodity X becomes bad as its consumption
increases beyond X1 unit and commodity Y becomes bad beyond the quantity Y1. This case is
represented in figure 16 where circular indifference curves are obtained. Point B represents the
highest satisfaction level that can be achieved under this condition therefore is the point of satiation
or bliss. In the figure, different shapes of indifference curves are shown in four possible situations.
In zone I both commodities are goods and thus the indifference curves have normal shape. In zone
II, commodity X is a bad and Y is a good and therefore the indifference curves are upward sloping.
In zone III, X is a good while Y is a bad and shown by upward sloping indifference curves. In zone
IV both the commodities are bad and thus indifference curves slope downward.

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[Figure 16: Different zones of goodness and badness and point of satiation]

When the consumer has excess of a commodity (so that it becomes a ‘bad’), his satisfaction can be
increased by reducing that ‘bad’. For example, the consumer can increase his satisfaction by
moving from point P to Q. In general, the preference direction of the consumer will be towards
point B i.e. the closer the consumer is to point B, the better off he is.

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8. Summary
 Indifference curve analysis is based on the concept of ordinal utility theory.
 In indifference curve analysis, consumer’s preferences are assumed to satisfy the properties
of completeness, transitivity and non-satiation.
 Indifference curves represent the same level of satisfaction or utility to the consumer and
are also called iso-utility curves.
 Marginal rate of substitution of X for Y (MRSX,Y) represents the amount of Y which the
consumer has to give up for the gain of one additional unit of X so that his level of
satisfaction remains the same.
 MRSX,Y diminishes as more and more of good X is substituted for good Y.
 The MRS between two goods is equal to the ratio between the marginal utility of two
goods.
 Indifference curves slope downward to the right.
 Indifference curves are convex to the origin.
 Two indifference curves cannot touch or intersect each other.
 Higher indifference curves represent higher level of satisfaction.
 Under certain circumstances, the shape of indifference curves may not be strictly convex
and downward sloping. Exceptional shapes of indifference curves can be obtained in case
of perfect substitutes, perfect complements and in the presence of ‘bads’ and neutral goods.


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