Let's build a simple model of rational vs. anchored valuation.
A rational investor should value a stock only by its fair valueV∗, based on fundamentals (cash flows, growth). But an anchored investor's perceived value V^ is a blend of the true value and the anchor A:
V^=(1−w)V∗+wA
Why this form? If the investor fully adjusted away from the anchor, weight w=0 and V^=V∗ (rational). If they were totally stuck on the anchor, w=1 and V^=A. Real people sit in between, so 0<w<1 is the "stickiness" of the anchor.
The decision error is:
ε=V^−V∗=w(A−V∗)
Why this step? Subtract the correct value from the perceived value. The error is proportional to (a) how sticky the anchor is, w, and (b) how far the anchor sits from truth, (A−V∗).
Rational updating uses Bayes' rule. For a hypothesis H ("this stock will rise") given evidence E:
P(H∣E)=P(E)P(E∣H)P(H)
Why this matters: A rational investor updates on all evidence E. A confirmation-biased investor filters the evidence set — they only let in evidence where P(E∣H) is high (supportive), and mentally discard disconfirming evidence.
So instead of updating on the full set {E1,E2,...,En}, they update only on the supportive subset. The result: their posterior belief P(H∣E) stays artificially high — the numerator keeps getting boosted, disconfirming evidence never lowers it.
Over-relying on the first number seen (e.g. purchase price, 52-week high) and adjusting away from it insufficiently.
What is confirmation bias?
Seeking/interpreting/remembering only evidence that supports what you already believe, ignoring contradicting evidence.
In the model V^=(1−w)V∗+wA, what does w represent?
The "stickiness" of the anchor — how much the perceived value is dragged toward the anchor A instead of the true value V∗.
What is the decision error caused by anchoring?
ε=w(A−V∗) — proportional to anchor stickiness and the distance of the anchor from fair value.
Why does confirmation bias inflate belief in Bayesian terms?
It filters evidence to only supportive signals (likelihood ratio > 1), so the posterior keeps rising and never gets corrected downward.
Best test to beat the purchase-price anchor?
"Would I buy this stock today with fresh cash?" If no, sell — your past price is irrelevant.
One antidote to confirmation bias before buying?
A pre-mortem: write the 3 strongest reasons the thesis could be wrong.
Why is anchoring an "efficient" but bad shortcut?
Fully adjusting a number is mentally costly, so the brain stops adjusting too early — fast but inaccurate.
Recall Feynman: explain to a 12-year-old
Imagine you buy a toy for 10.Latereveryonesaysit′sreallyonlyworth6. But because you *paid 10∗,yourbrainkeepswhispering"no,it′sworthlike8!" That sticky first number is anchoring. Now say you think this toy is the best toy ever — so you only listen to friends who agree and you ignore anyone who says it's junk. That's confirmation bias: you only collect "you're right!" and throw away "you're wrong." Together they trick you into keeping a bad toy way too long.
Dekho, do bahut common mental traps hain jo har investor phasata hai. Pehla hai anchoring — matlab tumhara dimaag pehle number ko pakad leta hai aur chhodta nahi. Jaise tumne stock 100pekhareeda,abwoh70 ka ho gaya, phir bhi dimaag bolta hai "yaar 85tohaihi!"—kyunki100 wala buy price ek "anchor" ban gaya. Reality yeh hai ki market ko pata bhi nahi tumne kitne ka khareeda; future value sirf fundamentals se decide hoti hai, tumhare purchase price se nahi.
Dusra trap hai confirmation bias — jab tum ek stock kharid lete ho, to sirf woh news padhte ho jo bolti hai "tum sahi ho." Bearish articles ko mute kar dete ho, debt footnote skip kar dete ho. Bayes ke terms mein, tum sirf supportive evidence ko andar aane dete ho, isliye tumhara belief artificially high rehta hai aur kabhi correct nahi hota. Phir jab bura news aata hai, tum totally blindsided ho jaate ho.
Simple model yaad rakho: V^=(1−w)V∗+wA. Yahan w anchor ki "chipakne ki taakat" hai. Jitna zyada w, utna zyada tumhari valuation galat — error hota hai w(A−V∗). Yeh dono biases milke tumhe losers ko hold karne pe majboor karte hain.
Ilaaj? Anchoring ke liye poochho: "Aaj agar mere paas cash hota, to kya main abhi yeh stock kharidta?" Agar nahi, to bech do. Confirmation bias ke liye pre-mortem karo — buy karne se pehle 3 sabse strong reasons likho ki tum galat kyun ho sakte ho. Yeh discipline hi tumhe emotional galtiyon se bachayega.
