Usually, stranger wariness is first noticeable at months

Stranger wariness is obvious when an infant no longer smiles at any friendly face & cries if an unaccustomed person moves too close, too quickly. Stranger wariness appears fleetingly around 6 months of age, though it is not always detected and at full force by 10 to 14 months.

industry had made the u.s. the richest nation in the world at this time. the second industrial revolution caused growth in industry and transportation, which allowed increased trade between nations.