On Nancy Pelosi’s 25th anniversary in Congress, I met with the authors of a new book that explains why so few female colleagues have joined her in all that time. Their findings are nothing to celebrate.

There are many reasons more women don’t run, including that they’re still less likely to come forward unless asked — and are less likely to be asked. They’re more put off by the possible nastiness and by the certain loss of privacy. What I did not know, though, is that incumbency doesn’t protect women in quite the same way it does men, according to “Women & Congressional Elections: A Century of Change,’’ by Barbara Palmer, of Baldwin-Wallace College and Dennis Simon of Southern Methodist University.

Nancy Pelosi is celebrating her 25th anniversary in Congress. But since Jeannette Rankin (R-MT) became the first woman elected to Congress in 1916, fewer than 10 percent of all members have been female. These days, 17 percent of those in Congress are women. (Chip Somodevilla/GETTY IMAGES)

Even after they’re elected to Congress, in other words, women continue to have more competition in both primary and general elections than their male counterparts do, and have to fight harder and raise more money to stay in office. Consciously or not, they’re perceived as more vulnerable, regardless of their margin of victory, say the authors, who’ve spent 14 years researching some 40,000 candidates.

Many of the challengers to incumbent congresswomen are female, too, in districts that have come to be seen as “women-friendly” — so that even if another woman does win, it’s a wash rather than a net gain in terms of female representation.

“You’re a lightning rod,’’ as a woman in office, Simon told me. And often, Palmer said, a reelection campaign turns into “a free-for-all.”

There’s more than a little something to that perception of certain districts as relative comfort zones for women candidates, though. In a chapter called “Demographics Is Destiny,” Palmer and Simon explain that it’s not too difficult to predict where women are most likely do well: in diverse, upscale urban areas with the most highly educated voters.

Sandra Fluke, a third-year law student at Georgetown University, with Rep. Elijah Cummings (D-MD), House Minority Leader Rep. Nancy Pelosi (D-CA), Rep. Carolyn Maloney (D-NY), and Rep. Eleanor Holmes Norton (D-DC) in February. Maloney and Pelosi represent two of the most “women-friendly” Congressional districts in the country. (Alex Wong/GETTY IMAGES)

I correctly guessed the two most women-friendly districts in the country, which are Carolyn Maloney’s, the old Silk Stocking district on the Upper East Side of Manhattan — what, no cookie? -- followed by Pelosi’s in San Francisco. But I would not have supposed that every one of the top 20 is currently held by a Democrat. And most of these districts — a whopping 75 percent --are in just two states, California and New York.

Mind you, only 8 of these 20 progressive havens are currently represented by women, which the glass-half-full researchers say might best be viewed as “areas of opportunity” when those seats do come open. (Seven of the 12 men who represent them are near retirement age, the authors note hopefully.)

Where women have the hardest time getting elected is in rural or blue-collar areas, which helps explain why even some blue states, like New Jersey and Massachusetts, elect so few women.

The number of women Republicans in Congress did grow in 2010, from 17 to 24, but that’s still far paltrier than across the aisle, where there are 49 Democratic women.

And of the 20 districts least likely to elect a woman, 19 are Republican, and all are in the South. The only Democrat on the list, Mike Ross, who represents the 4th Congressional District in Arkansas, is not running again.

Texas has the most of these male bastions, with four; there’s also particularly rough terrain for women in Tennessee, South and North Carolina, Louisiana, Georgia, Arkansas, Alabama, Mississippi and Virginia.

The safest district of all for men? There’s a tie for that distinction, between Georgia’s 9th and Alabama’s 4th Congressional Districts, where data suggests a woman candidate has about a 2 percent chance of winning.

Amid so much documentation that women candidates are still held to a higher standard by both voters and reporters, there is this note of encouragement at the book’s end: With the country becoming more urban, more diverse, more educated (I’m not so sure about that one) and less blue-collar, big gains could be ahead.

But for Republican women in the rural South, where’s the happy ending? Probably not in the House, this book suggests. At least, not any time soon.