Try a small experiment before reading on. Write down the last two digits of your Social Security number (or your phone number), treat them as a dollar price, and ask yourself: would you buy this bottle of red wine at that price? Then write down the most you would actually pay.

Think that number couldn't possibly matter? Ariely, Loewenstein & Prelec (2003) had 55 MIT Sloan MBA students do exactly this: first write the last two digits of their Social Security number (SSN) and answer 'would you buy at this price,' then submit real, binding maximum bids for 6 products (a cordless keyboard, a cordless trackball, wine, Belgian chocolates, design books — average retail around $70), using the Becker-DeGroot-Marschak mechanism, where transactions really happen. The result: people whose SSN's last two digits were above the median bid 57%–107% more than those below it; by quintiles, the top quintile typically bid about 3 times the bottom quintile — $56 vs $16 for the same cordless keyboard. The correlation between SSN and bids ran 0.32–0.52, significant for all six product categories. An anchor doesn't have to come from someone else: an irrelevant number you merely 'brought to mind' can price goods for you — and real money on the line doesn't stop it.

The granddaddy of this trick is Tversky & Kahneman's (1974) wheel-of-fortune experiment: a wheel marked 0–100 (rigged to stop only at 10 or 65) was spun in front of subjects, who were first asked whether the percentage of African nations among UN members was higher or lower than that number, then asked for their own estimate. The group that drew 10 gave a median estimate of 25%; the group that drew 65 said 45% — a number generated at random, in plain sight, with zero relevance to the question, steered the judgment. Paying subjects for accuracy did not weaken the effect.

'That's just clueless students'? Northcraft & Neale (1987) had professional real-estate agents (n=47) and business students (n=54) tour the same house in person (appraised at $135,000 a year earlier); the only difference was the listing price in the packet, set at $119,900 / $129,900 / $139,900 / $149,900. The experts' average valuation was pulled from $114,204 all the way to $128,754 — anchored about as strongly as the amateurs. The stinger came in the debrief: only 19% of the experts admitted considering the listing price (vs 37% of the amateurs), and just 8% ranked it among their top three considerations. Wilson et al. (1996) delivered the final blow: explicitly warning people to guard against the number's influence did not eliminate anchoring either.