“We asked partners to pinpoint the factors underlying the pay differences,” said Jeffrey A. Lowe, who heads Major, Lindsey’s law firm business, “and the No. 1 factor was origination,” or who receives credit for bringing a legal matter to the firm.

Rainmaking, or attracting legal work from clients, has always been a top factor in lawyer earning power, and is continuing to grow. The average origination amount, according to the survey, was almost $2.5 million, and that amount was up 25 percent from two years ago. Typically, lawyers’ annual compensation is tied to the amount of business they bring.

“We found that, predominantly, a partner’s compensation is tied to bringing in business to the law firm,” Mr. Lowe said.

Women partners, the report found, brought in an average of $1.7 million worth of business compared with the $2.6 million average of their male counterparts. Because there are more male partners, the average skews higher than if there were equal gender representation. Two years ago, women brought in $1.2 million worth of business, and male counterparts chalked up $2.2 million.

The lag in pay for female partners in either attracting business or getting credit for it could stem from several factors. One is that the “old boys network” still has an outsize influence because of connections made in law school or earlier that affect who is hired to handle their corporate legal matters.