Behavioral Finance Lecture 03: Debunking CAPM and conventional Behavioral Finance

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CAPM still dom­i­nates the teach­ing of finance, but it was always non­sense because it relied on the fol­low­ing two assump­tions:

In order to derive con­di­tions for equi­lib­ri­um in the cap­i­tal mar­ket we invoke two assump­tions.

First, we assume a com­mon pure rate of inter­est, with all investors able to bor­row or lend funds on equal terms.

Sec­ond, we assume homo­gene­ity of investor expec­ta­tions:

investors are assumed to agree on the prospects of var­i­ous investments—the expect­ed val­ues, stan­dard devi­a­tions and cor­re­la­tion coef­fi­cients described in Part II.”

Even Sharpe had to instant­ly admit that, as assump­tions go, these two were doozies: he con­tin­ued that “Need­less to say, these are high­ly restric­tive and undoubt­ed­ly unre­al­is­tic assump­tions.” But nev­er let real­ism get in the way of a neo­clas­si­cal the­o­ry! He defend­ed this non­sense with a twist­ed appeal to Mil­ton Fried­man’s “instru­men­tal­ist” method­ol­o­gy:

How­ev­er, since the prop­er test of a the­o­ry is not the real­ism of its assump­tions but the accept­abil­i­ty of its impli­ca­tions,

and since these assump­tions imply equi­lib­ri­um con­di­tions which form a major part of clas­si­cal finan­cial doc­trine,

it is far from clear that this for­mu­la­tion should be rejected—especially in view of the dearth of alter­na­tive mod­els lead­ing to sim­i­lar results.” (Sharpe 1964, pp. 433–434)

A major facet of CAPM was using the con­cept of Expect­ed Val­ue devel­oped by John von Neu­mann and Oskar Mor­gen­stern in The­o­ry of Games and Eco­nom­ic Behav­ior. As is so often the case in eco­nom­ics, their work was mis­in­ter­pret­ed. While CAPM involved blend­ing neo­clas­si­cal indif­fer­ence curve analy­sis of indi­vid­ual behavior–which I debunked in the first lec­ture in this series–with von Neu­man­n’s Expect­ed Val­ue analy­sis, von Neu­man­n’s inten­tion was to devel­op a numer­i­cal mea­sure of util­i­ty and elim­i­nate indif­fer­ence curves from eco­nom­ic the­o­ry.

Despite these obvi­ous rea­sons to dis­miss CAPM, it took over the pro­fes­sion, and part of the rea­son for its suc­cess was its appar­ent close fit to the data at the time it was devel­oped. But was this just a fluke, giv­en the tran­quil eco­nom­ic times dur­ing which CAPM was devel­oped?

Even Behav­ioral Finance, which is crit­i­cal of CAPM, has large­ly been devel­oped on a mis­un­der­stand­ing of von Neu­mann. It applies the sub­jec­tive con­cept of util­i­ty and expect­ed returns to argue that there are numer­ous “para­dox­es” where peo­ple don’t behave “ratio­nal­ly” giv­en a finan­cial choice, and that these “para­dox­es” can’t be explained by risk aver­sion, loss aver­sion, or all oth­er man­ner of con­ven­tion­al expla­na­tions.

In fact all these para­dox­es dis­ap­pear if objec­tive prob­a­bil­i­ty is used, which is what von Neu­mann and Mor­gen­stern insist­ed upon–literally–in their book:

Prob­a­bil­i­ty has often been visu­al­ized as a sub­jec­tive con­cept more or less in the nature of esti­ma­tion. Since we pro­pose to use it in con­struct­ing an indi­vid­ual, numer­i­cal esti­ma­tion of util­i­ty, the above view of prob­a­bil­i­ty would not serve our pur­pose. The sim­plest pro­ce­dure is, there­fore, to insist upon the alter­na­tive, per­fect­ly well found­ed inter­pre­ta­tion of prob­a­bil­i­ty as fre­quen­cy in long runs.” (von Neu­mann & Mor­gen­stern 1944, p. 19)

Final­ly, econ­o­mists nor­mal­ly deride the “pay­back peri­od” means of decid­ing between invest­ments, on the argu­ment that this ignores the time val­ue of money–an argu­ment you’ll find in the Wikipedia entry on this top­ic. But John Blatt showed in in the 1980s that the pay­back peri­od takes account of both the time val­ue of mon­ey and uncer­tain­ty about the future, at least to a rudi­men­ta­ry lev­el.

Here are the two Pow­er­point Files for this lec­ture (Part 1; Part 2). I also neglect­ed to upload the Pow­er­point file for lec­ture 2; here they are (Part 1; Part 2). I’ll also edit the post the include them.

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About Steve Keen

I am Professor of Economics and Head of Economics, History and Politics at Kingston University London, and a long time critic of conventional economic thought. As well as attacking mainstream thought in Debunking Economics, I am also developing an alternative dynamic approach to economic modelling. The key issue I am tackling here is the prospect for a debt-deflation on the back of the enormous private debts accumulated globally, and our very low rate of inflation.