Lead was historically added to paint because it improved durability, drying characteristics, and color performance.
The federal government banned consumer use of lead-containing paint above the applicable threshold for residential applications in 1978, which is why pre-1978 housing receives particular attention under federal lead regulations.
However, 1978 should not be viewed as a guarantee. Not every pre-1978 house contains lead-based paint, and different painted components within the same house can have very different lead concentrations.
Testing is the best way to determine whether a particular painted surface contains lead.