5 ideas
Mackay argues that people think in herds and go mad in herds, but recover their senses slowly and one by one. Contagion spreads delusion quickly because each person's belief is reinforced by everyone else's. Disillusionment is private and staggered, so a mania can outlast the evidence against it and the collective damage compounds before enough individuals defect.
In 1630s Holland, rare bulbs such as the Semper Augustus changed hands for the price of houses. Traders bought and sold promises on bulbs still in the ground that they never meant to plant. Mackay tells of a sailor who ate a costly bulb, mistaking it for an onion. When buyers began refusing to honor contracts in 1637, prices collapsed almost overnight because nothing but continued buying had supported them.
One sought subscribers for 'an undertaking of great advantage, but nobody to know what it is' and reportedly collected thousands of pounds in a morning before its founder vanished. At a mania's peak, investors stop evaluating the underlying venture and buy only because they expect someone else to buy from them at a higher price.
John Law's Mississippi scheme in France tied the issue of banknotes to shares in a trading company. Each fed the other, and prices spiraled far beyond any real colonial revenue. When holders tried to convert paper back into coin, the state responded with edicts limiting how much gold and silver citizens could own. Those coercive attempts to prop up confidence signaled desperation and hastened the collapse.
Mackay shows that witch persecutions were sustained by legal machinery, not just popular fear. Torture produced confessions and confessions named new suspects. Paid witch-finders like Matthew Hopkins had incentives to find more witches. When courts stopped admitting such evidence, accusations fell away, which reveals that the 'proof' of the menace was generated by the system prosecuting it.