If you just go by what's under the limelight, U.S. billionaires are a boys club composed of trendy young tech-heads and wily old investors who have been plying their trade for decades. But look on the margins and you can find female billionaires in the U.S.

They are, however, an exclusive and low-profile group -- perhaps even more so than "average" billionaires. Of the 158 U.S. citizens listed in the Bloomberg Billionaires Index of top 500 billionaires, only 30, or 19 percent, are women. Only three members of the top 15 richest U.S. women could be considered self-made billionaires, with the rest inheriting their wealth from fathers, grandfathers and husbands of family dynasties that have seen billions pass through several generations.

Indeed, the family dynasties associated with just three companies -- Walmart, Mars and Cox Enterprises -- produced the wealth for the majority of the top 15 of U.S. female billionaires, with the top person on the list, Alice Walton, being one of three living heirs to the family fortune derived from Walmart. The Mars candy and food company, meanwhile, has five women in the top 15, including four sisters.

The following 15 women, in ascending order, are the nation's richest. The number in paranthesis is where they placed on the overall Bloomberg Billionaire Index as of March 20:

15. (296) Margaretta Taylor, $5.90 billion
Taylor owns 16 percent of Cox Enterprises, a share she inherited from her mother, Anne Cox Chambers, the daughter of Cox Enterprises founder James M. Cox. Neither Margaretta nor her siblings or mother have ever had an active role in the company, according to Forbes.

 

14. (250) Ingrid Wu, $6.61 billion
Wu, 69, co-founded AAC Technologies Holdings, a mobile phone component maker with revenue of $2.3 billion in 2016, with her husband Pan Zhengmin in 1993, according to Bloomberg News. She served as chief financial officer of the Shenzhen-based company, which builds miniaturized speakers and receivers for clients including Apple and Samsung, according to Bloomberg.

 

13. (248) Marian Ilitch, $6.64 billion
Ilitch, 85, is the she second-richest self-made woman in the U.S. and, perhaps, the most visible U.S. female billionaire. She is the co-founder, with her late husband Mike Ilitch, of Little Caesers Pizza, the third-largest pizza chain in the U.S. Her success with the pizza company springboarded her to success in other industries, including ownership of the Detroit Tigers baseball team and Detroit Red Wings hockey team, which has won four Stanley Cup championships under her ownership. She has also become a player in the casino industry and was one of the original owners of MotorCity Casino in Detroit.

 

12. (247) Christy Walton, $6.70 billion
Christy Ruth Walton, 69, inherited her wealth after the death of her husband John T. Walton, son of Walmart founder Sam Walton. She used to be considered the richest woman in the U.S., but an investigation by Bloomberg News in 2015 found that most of of John T. Walton's estate was left to her son, Lukas Walton. In 2008, the World Giving Index ranked her as the most generous female philanthropist in the U.S. based on the percentage of her wealth she gave to charity.

 

11. (212) Diane Hendricks, $7.59 billion
Hendricks, 70, is the richest self-made woman in the U.S.—the daughter of a Wisconsin dairy farmer who, with her late husband Ken Hendricks, built a roofing contracting business, ABC Supply, into the largest wholesale distributor of roofing materials in the U.S. She is also a movie producer and a major donor to Republican candidates, including Wisconsin Gov. Scott Walker. She serve as an economic advisor to the presidential campaign of Donald Trump.

 

10. (211) Valerie Mars, $7.62 billion
One of four sisters who share a spot on the billionaire index, Valerie Mars is senior vice president and head of corporate development at her family's candy and food conglomerate, where she started working in 1992. She graduated from Yale University and received her MBA from Columbia Business School. She sits on the board of directors of Fiat Chrysler Automobiles N.V. According to her biography on the car company's website, she serves on the Board of Conservation International and is a director emeritus of The Open Space Institute.

 

9. (210) Pam Mars-Wright, $7.62 billion
Mars-Wright, 57, served as Mars chairwoman from 2004 to 2006. "It's good for a board to rotate the personnel so it doesn't end up being the same old thing all the time," she told Campden FB in 2006. "I personally don't believe that having the same thing for 25 years is necessarily a good thing." She is a graduate of Vassar College.

 

8. (209) Victoria Mars, $7.62 billion
Like her sisters, Victoria Mars started her career working at the Mars company. She has served on the company's board of directors since 2006 and served as company chairman in 2014. A graduate of Yale University, she earned an MBA from the University of Pennsylvania's Wharton School of Business. She also attended the Foxcroft college-preparatory boarding school in Middleburg, Va., and now is a member of the school's board of trustees.

 

7. (208) Marijke Mars, $7.62 billion
Mars, 53, is among the sisters who have been made rich by the global food and candy conglomerate, inheriting an 8 percent stake in the privately held company when her father Forrest Mars Jr. died in 2016, according to Forbes magazine. A graduate of Duke University, she has worked in management at the Mars company.

 

6. (165) Blair Parry-Okeden, $8.92 billion
Parry-Okeden, 67, is the daughter of the late billionaire Barbara Cox Anthony and the grandaughter of the late James M. Cox. James Cox, a 1920 Democratic presidential candidate, founded a chain of newspapers in 1898 that grew into the media company Cox Enterprises, the third-largest cable operator in the U.S. Parry-Okeden inherited a 25 percent share of the company, which had sales of $20.4 billion in 2017. Known for keeping a low profile, she moved to Australia in the 1970s and is active in philanthropy.

 

5. (130) Abigail "Abby" Johnson, $10.90 billion
Johnson, 57, is the president and CEO of Fidelity Investments, the company her grandfather Edward C. Johnson II founded in 1946. The Johnson family reportedly still owns 49 percent of Fidelity. The Wall Street Journal reported in 2005 that Abby Johnson was once one of the largest single shareholders in the company before selling a significant portion of her holdings that year, allegedly after a dispute with her father over the direction of the company. She is also CEO of FMR, the parent of Fidelity Investments, and owner of family assets including a tomato farm in Maine and much of the Seaport section of South Boston, according to Bloomberg News.

 

4. (74) Elaine Marshall, $16.2 billion
Marshall, 75, was married to Everett Pierce Marshall, an oil industry executive and owner of 16 percent of Koch Industries, the Wichita, Kan.-based conglomerate with operations in the oil, commodities and paper pulp sectors. Elaine Marshall and her two sons inherited his wealth after his death in 2006. Her father-in-law, the billionaire J. Howard Marshall, became a tabloid celebrity when he married the late actress/model Anna Nicole Smith in 1994. After J. Howard Marshall's death in 1995, Smith waged a court battle with the Marshall's family, including Elaine Marshall, over his estate. She sits on the Koch Industries board.

 

3. (57) Laurene Powell Jobs, $18.4 billion
She was the wife of the late Steve Jobs and inherited his wealth, including stakes in Apple and Walt Disney, when he died in 2011. Jobs, 54, is active in a number of social and environmental causes and is the founder and president of the Emerson Collective, a supporter of "education, immigration reform, the environment, and other social justice initiatives," according to the organization's website. The Emerson Collective recently became the majority owner of The Atlantic magazine and, bucking a trend that has seen a decline in U.S. news print publishing, announced plans for a major expansion of staff.

 

2. (24) Jacqueline Mars, $30.6 billion
Mars, 78, is the granddaughter of Frank C. Mars, who in 1911 founded the Mars Inc. candy company in Newark, N.J. She is among a number of Mars family dynasty members who have inherited vast wealth due to the success of the privately held company, which is entirely owned by the Mars family. She graduated from Bryn Mawr College in Pennsylvania with a degree in anthropology in 1961. In 2013, she was convicted of reckless driving and fined $2,500 after her card swerved over the center line on a Virginia highway and struck a minivan, causing the death of an 86-year-old woman. Mars told a witness she fell asleep at the wheel, according to news reports.

 

1. (18) Alice Walton, $41.1 billion
Walton, 68, is the daughter of Sam Walton, the founder of retail giant Walmart, and one of three living heirs to the Walton family fortune. A graduate of Trinity University in San Antonio, Texas, she had a long career in the financial industry as an investment banker, analyst and investment manager. She is an avid art collector and is known for applying the same focus on artwork as she did on investments during her financial career. "What distinguishes Ms. Walton from many other vastly wealthy collectors is that she carefully does her homework," the New York Times wrote in 2005. Alice Walton financially backed Hillary Clinton--a former member of the Walmart board of directors--in the 2016 presidential election campaign after a long history of supporting Republican candidates.