What is put call parity in options trading? How is put call parity calculated? Why does it matter?
Put Call Parity  Definition
Put Call Parity is an
option pricing concept that requires
the
extrinsic values of call and put options to be in equilibrium so as to prevent
arbitrage. Put Call Parity is also known as the Law Of One Price.
Put Call Parity  Introduction
Put Call Parity is a concept identified by Stoll in 1969, that defines the relationship that must exist in
European call and put options. Put options,
call options and their
underlying stock forms an interrelated securities complex in which the combination of any 2 components
yields the same
profit/loss profile as the 3rd instrument. Under this kind of complex relationship, no combination of 2 components should
yield a position with an asymmetric profit/loss profile as the 3rd instrument so that balance is maintained in the system. This balance is known
as the Put Call Parity in option trading. The concept of Put Call Parity is especially important when trading
synthetic positions. When
there is a mispricing between an instrument and its synthetic position, an
options arbitrage opportunity exists.
Put Call Parity requires, mathematically, that option trading positions with similar payoff or risk profiles (i.e Synthetic Positions)
must end up with the same profit or loss upon expiration such that no arbitrage opportunities exist.
An example of Put Call Parity is in terms of
Fiduciary Calls
and
Protective Puts. Fiduciary Calls and Protective Puts have the same
profit/loss profile that you see below.
If the underlying stock remains completely stagnant by expiration of both positions, both positions would decline in value equal to
the extrinsic value (
premium) paid on the options. Neither option trading positions would have any advantage over the other during
expiration.
Put Call Parity  An Illustration
To illustrate Put Call Parity at work in option trading, figures are put into both the Fiduciary Calls and Protective Puts positions below:
Assume : XYZ shares trading at $50 on 1 Jan. XYZ Feb50Call and Feb50Put are asking at $2.00
Fiduciary Calls : Buy 1 contract of Feb50Call for $200
Protective Puts : Buy 100 Shares of XYZ and 1 contract of Feb50Put for a total of $700
During expiration of the Feb options, XYZ is still trading at $50
Fiduciary Calls : Lose the whole $200 as the Feb50Calls expire worthless. Net Loss = $200
Protective Puts : Lose the whole $200 on the Feb50Puts while the XYZ shares remain stagnant. Net Loss = $200

Notice above where Put Call Parity exists, the net loss on both positions are exactly the same so that there are no arbitrage opportunity to be
exploited by being
long one position and
shorting the other simultaneously.
Here is another illustration of Put Call Parity at work using the above example.
Assume : XYZ shares trading at $50 on 1 Jan. XYZ Feb50Call and Feb50Put are asking at $2.00
Fiduciary Calls : Buy 1 contract of Feb50Call for $200
Protective Puts : Buy 100 Shares of XYZ and 1 contract of Feb50Put for a total of $700
During expiration of the Feb options, XYZ is trading at $60.
Fiduciary Calls : Profit = [($60  $50)  $2] x 100 = $800
Protective Puts : XYZ shares gain = ($60  $50) x 100 = $1000, less loss on put options = $200, net profit = $800

Notice above that when Put Call Parity exists, the profit on both positions are also exactly the same. However, the problem is, Put Call Parity
rarely exist across the board.
Put Call Parity  The Reality Of Put Call Disparity
As you might have noticed above, Put Call Parity requires that the extrinsic value of call and put options of the same strike price to be the
same. However, in reality, the extrinsic value of put and
call options are rarely in exact parity in option trading even though
market makers have been charged with the
responsibility of maintaining Put Call Parity. When the outlook of a stock is bullish, the extrinsic
value of call options tend to be higher than put options due to higher
implied volatility
and when the outlook of a stock is bearish, the extrinsic value of put options tends to be
higher than call options.
In the above examples, when the outlook of XYZ stock is bullish, the call options would have a higher extrinsic value than
put options, resulting in a higher loss for the Fiduciary Calls than Protective Puts if XYZ shares remain stagnant. In this case, an option arbitrage
opportunity would present itself by going long the Protective Puts and going short the Fiduciary Calls. If Fiduciary Calls and Protective Puts
are established over a significantly long period of time, the cash reminder of the Fiduciary Calls would also have returned a risk free rate of
return, resulting in an arbitrage. However, such option trading arbitrage opportunities are
rare even under Put Call Disparity as the presence of bid/ask spreads and commissions tends to neutralise gains from such disparities. In fact,
the empirical studies of Kamara and Miller in "Daily and Intradaily Tests of European PutCall Parity" (JFQA, 30, 1995, No. 4, 519539) also
concluded that even though there are many violations to the Put Call Parity theorem in practise, most of the option trading arbitrage opportunities result in
a loss when transaction costs and delays in execution are accounted for.
Deviations from Put Call Parity or Put Call Disparity has also been used in the analysis of future stock prices as stocks with relatively expensive
calls have been found empirically to outperform stocks with relatively expensive puts by a measurable significance and margin.
Deviations from Put Call Parity can also lead to profitable arbitrage opportunities using
options arbitrage strategies such as a
Box Spread if the disparity is significant enough.
Put Call Parity Theorem  Calculating Put Call Parity
The mathematical representation of Put Call Parity in the case of Fiduciary Calls and Protective Puts is:
C + X / (1+RFR)
^{t} = S + P
Where : c = Call premium, X / (1+RFR)
^{t} = Present value of strike price, S = initial stock price, P = Put premium
Each of the securities in the Put Call Parity Theorem can thus be expressed as:
Stock Price = C  P + X / (1+RFR)
^{t}
Put Premium = C  S + X / (1+RFR)
^{t}
Call Premium = S + P  X / (1+RFR)
^{t}
Present Value Of Strike Price = S + P  C
The above relationships assumes that no dividends are being paid. The Put Call Parity Theorem for dividend paying stocks is:
P = C  S + x/(1+RFR)
^{t} + d/(1+RFR)
^{t}
C = P + S  x/(1+RFR)
^{t}  d/(1+RFR)
^{t}
Where d = Dividend To Be Paid
Put Call Parity  Limitations
Put Call Parity applies mainly to European style options as
American Style options allows early exercise which can result in
profit opportunities that lies beyond the Put Call Parity Theorem.
Put Call Parity  Assumptions
The assumptions under the Put Call Parity Theorem are:
1. Constant Interest Rate
2. Future dividends are known for sure