Final answer:
Psychological accuracy became a major guiding principle in drama, leading to realistic costuming.
Step-by-step explanation:
Realistic costuming took over in drama when psychological accuracy became a major guiding principle. In the late 19th century, there was a move towards Realism in theatre, which led to a focus on the psychology of characters and how to best communicate information about time and place to the audience. With these developments, the modern Costume Designer was born.