July 1st, 2007, 6:59 am
The argument is no arbitrage. If S-pre<S-post + dividend then buy S and you win S-post+dvidend-S-pre>0, otherwise sell S.Of course, you can say that this varies each time, but on average S-pre=S-post+dividend by this argument.Actually, the price will be reduced by the post-tax value of the dividend, but this does not change the principle. Empirical evidence is relatively poor, because the dividends most often are quite small compared to the market capitalisation of a company. This way the effect is dominated by the offer-demand noise.