The hot eyewear look of 2008 was pretty much defined by plastic Wayfarer knockoffs, garish neon trapezoids often seen color-coordinated with a plaid shirt and sneakers. Or else it was “shutter shades,” those ventilated blinders popularized by Kanye West.

This summer, however, the memo for sunglasses says circles are in. Very round shapes, as round as goggles in some cases, appeared in the recent women’s collections of Marc Jacobs and Proenza Schouler; and, for men, from Ralph Lauren, Zegna and Lanvin (most costing from $300 to $400). Last month, New York magazine included round sunglasses among the anticipated trends of the summer. And, as predicted, they are already appearing on the streets in numbers not seen since the release of the last Harry Potter book.

1997 Harry Potter inspires children around the world to reconsider the nerd factor of specs. Photo: Peter Mountain/Associated Press

While most fashion trends — and especially this one — are circular, round sunglasses, seemingly everywhere all at once, provide a case study of the group-think mentality of the fashion industry. There are even inexpensive examples ($10 to $11) at Urban Outfitters and Fred Flare.

Given that there is no obvious source for the revival and that typically it takes more than a year to develop expensive sunglasses from a design to prototype to salable object, how could it be that all of these designers stumbled upon the same idea at the same time?

“This is what fashion is,” said Simon Jablon, the English designer behind the Linda Farrow sunglasses label, which was founded by his mother in 1970. “It is a trend. You can just sense it. You have a feeling for where things are going to go.”