The Peroxide Effect (or Kharasch Effect) occurs only with $HBr$ (not $HCl$ or $HI$).

• In the presence of organic peroxides, the addition of $HBr$ to an alkene follows an Anti-Markovnikov pathway.
• Mechanism: This is a free radical mechanism. The peroxide generates $B{r}^{.}$ radicals. The bromine radical adds to the less substituted carbon to form a more stable substituted carbon radical intermediate.
• Example: Propene $+HBr+\text{peroxide}\to$ 1-Bromopropane.