In 1969, the dairy industry launched an advertising campaign with the slogan, "Every body needs milk."
In Oregon, the marketing team decided to conduct an experiment to find out whether showing more skin on a billboard would attract more attention. To do this, they created two different versions of an image. Both showed an attractive young woman lying down, feeding milk to a kitten. But in one version she was wearing slacks and a long-sleeved blouse. In the other, she was wearing a bikini.
It took me a lot of searching, and I wasn't able to find very good-quality copies, but I believe these are the two different billboard scenes:
Minneapolis Star - Feb 20, 1970
So, did one billboard attract more attention than the other? The marketers surveyed 231 teenagers and concluded that there was "no indication that the amount of clothing made any difference in the awareness."
Salem Capital Journal - May 6, 1970
That was their conclusion, but I'm not sure I believe them, because the rest of the marketing campaign focused heavily on bikini-clad models. Two examples below.
They even made it possible to buy the bikini-themed images as a poster and towel. Which suggests the bikini billboards did attract more attention.
Oakland Tribune - May 24, 1970
The Capital Journal - June 3, 1970