The shift hasn't been lost on retailers. Several, including Lululemon Athletica, Coach and Michael Kors, have begun acting on the opportunity for growth in menswear after investing more heavily in womenswear for years.
"I think we're in an era where every shopper counts, and retailers are working hard to squeeze growth out of shoppers, and you can't count on Mom with the big basket anymore," Corlett said. "You have to look at every shopper in the store."
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Total men's U.S. apparel sales are about $58 billion compared to about $112 billion for women's, according to the NPD Group, a market research firm. Although men's had been growing more briskly than women's during the year ended in July 2012, they both grew 2.4 percent for the 12 month period ending in July 2013.
Going after the guys
Michael Kors is trying to boost menswear. Its current business is relatively small, and driven by sportswear. During the first-quarter earnings conference call on Aug. 6, Kors CEO and Chairman John Idol said expect the category to accelerate its growth over the next few years.
Watches and leather goods are the other cornerstones of the men's business, Idol added. This trio of businesses could eventually become a more than $1 billion business as it expands its shop-in-shops in men's sportswear and men's leather goods, he added.
"So we are planting the seeds, we're making investments, and it's going to take us time, but we are very serious about the men's business and the opportunity it holds for our company," he said.
Rival Coach is also betting on the menswear business by opening more dual gender locations globally and expanding its men's selections in existing North American retail stores.
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So far, the move appears to be paying off. In fiscal year 2013, Coach's men's business grew nearly 50 percent to about $600 million.
"Looking ahead, we remain excited about the prospects to our global men's business where we believe we can reach $1 billion in sales in three years from the $600 million achieved last year," said Coach CEO Lew Frankfort on its fourth-quarter earnings call July 30.
To reach this male audience, retailers often need to adjust their strategy.
Cortlett emphasized the need for retailers to make shopping more efficient so guys don't feel like the experience is tedious.
Having readily available staff helps. More men than women ask for assistance from a sales associate, according to WSL's findings—shattering the myth that men don't ask for directions (or at least they do in the retail arena).
And it might seem like a tough sell to convince men to shell out $100 for yoga apparel, but the bet is paying off. Speaking to analysts on Sept. 12, outgoing Lululemon CEO Christine Day emphasized the need to recognize "the unique characteristics of our male guests."
Lululemon saw "tremendous growth" in the last quarter, after more color was injected into its menswear line and more "technical" items such as clothing with wicking fabric were added back into its product assortment.
In Lululemon's most recently ended quarter, roughly 13 percent of its total revenue came from men's sales. Next up, the company is scouting for locations for standalone men's stores, which it plans to open by 2016.
—By CNBC's Katie Little. Follow her on Twitter