What makes a fighter? Is it nature? Nurture? Or a combination of both? No one knows for sure, and sibling fighters only confuse the issue further.
In MMA's 19-year history, there have been 20 sets of siblings that have competed at the highest levels—in the UFC, Pride, Strikeforce or all three.
Sometimes both brothers have had significant success. Sometimes, one has stood out, and the other comes along for the ride.
Some familiar names grace this list. From the Hughes to the Gracies, from the Nogueiras to the Serras, MMA's most famous families are all included.
Which brothers stand out from all the others as the top family affair in MMA?
Read on to find out.
Big Show Appearances (X2): 5
Jared Rosholt was a darn good wrestler at Oklahoma State. A three-time All-American, he came within a single point of winning a 2010 national title. Despite that success, he lived in the shadow of his brother Jake, a three-time NCAA champion.
The two are among the top prospects in the whole sport. When they get the call, the Rosholts will be ready to take the UFC by storm.
Big Show Appearances (X2): 3
Just 30 years old, Keith Wisniewski comes from a different time and place: When an MMA fighter could make his professional debut at just 17, and no one would blink an eye. When it took 30 fights on the regional scene just to get a shot at the UFC. When a loss would relegate you to six years on the sidelines waiting for another shot.
His brother Justin never got a shot in the Octagon, but he stayed close to the sport, promoting the Duneland Classic with Keith.
Big Show Appearances (X2): 16
Matt Serra is one of MMA's most unlikely success stories. His career seemed all but over when he won the fourth season of The Ultimate Fighter and an automatic title shot against welterweight champ Georges St-Pierre—Serra pulled off the miracle upset and wrote his name into the history books.
His younger brother Nick is an accomplished grappler but has had an undistinguished career in MMA. The sole highlight? His awesome nickname "the Mad Monkey."
Big Show Appearances: 15
Rashad Evans needs no introduction: Immediately a polarizing figure in the sport thanks to a star turn on The Ultimate Fighter, Rashad is a former champion and an amazing athlete.
His brother Lance didn't have the same success. He never made it to The Ultimate Fighter house, losing in a "fight in" bout to jiu-jitsu wiz Vinny Maghaeles.
But Lance continues to fight: his latest enemies are terrorists, and his fight isn't in the Octagon—it's overseas with the U.S. military.
Big Show Appearances (X2): 13
Enson Inoue would rather lose and learn something about himself as a man than win by taking the easy route. This philosophy made him one of the sport's most interesting figures in the early days, if not one of the most successful.
His older brother Egan has 21 career fights, all either in Hawaii or Japan. His resume includes fights in Pride, Shooto and Superbrawl.
Big Show Appearances (X2): 4
Tyrone Roberts made a single UFC appearance, was victorious, and never made the big show again. It's a shame—he was a fine middleweight.
His big brother Andre was an enormous heavyweight from an era when it was perfectly acceptable to wear a t-shirt into the cage.
Big Show Appearances (X2): 13
The Miller brothers are the living embodiment of the power of The Ultimate Fighter. The two sport almost identical records, but Cole is by far the more famous.
The difference? Micah didn't make it into the reality show house, and Miller had a memorable run.
Big Show Appearances (X2): 8
Lightweight Pat Healy has taken Strikeforce by storm, going 5-1 in the promotion. His only loss? Former champion Josh Thomson.
While his brother Ryan hasn't had the same level of success, his 23 wins make him a more than respectable complement to his sibling.
Big Show Appearances (X2): 17
The other branch of the Gracie family with notable success. (Helio's sons will come along a little later on this list.)
These grandsons of Carlos Gracie, founder of Gracie Jiu-Jitsu, weren't as influential as their cousins, but nobody fought with more intensity or passion.
Big Show Appearances (X2): 16
While little brother Dan tries to fight his way back to the UFC, Joe continues to be a mainstay in the promotion.
Big Show Appearances (X2): 22
These two rugged New Jersey-ites—men who aren't afraid to rock a beard or some Springsteen—are tough and scrappy fighters. Lightweight Jim is the more successful of the two, coming a fight away from a title shot last year.
Big Show Appearances (X2): 27
Without Rickson, Royler and Royce, there might not be mixed martial arts in either America or Japan. Their legacy is unquestionable and unassailable.
Big Show Appearances (X2): 19
Clay is one of the UFC's most popular fighters, a whirling dervish of flying hair and flying fists.
His brother Jason hasn't had the same level of success—but he has managed 19 wins to go with his 26 losses.
Big Show Appearances (X2): 26
What is it about two twins, virtually identical in nature and nurture, that would make one driven to be a champion fighter and the other happy to hang back on the farm?
Life, friends: it is a mystery.
Big Show Apearances (X2): 33
"Ninja" Rua had it all: the amazing run in Pride, the incredible yet generic nickname. Who would have imagined his little brother would usurp him on both counts?
Yet "Shogun" has done just that, winning the UFC light heavyweight title and giving his body and soul in a heart-wrenching loss to Dan Henderson that ranks among the best MMA bouts of all time.
Big Show Appearances (X2): 28
Ken's career was cut short by a run in the WWE. Frank retired on top as the UFC champion. Otherwise, these two superlative fighters would have likely topped the list.
The two adopted brothers, for what it's worth, hate each other.
Frank left Ken's Lion's Den when his older brother threw a computer monitor at him.
They were rumored to be potential opponents for years, but the Cain vs. Abel-inspired fight never came to pass.
Big Show Appearances (X2): 33
Fedor is the best and most accomplished heavyweight in MMA history.
His brother saw his career shortened, at least internationally, by undisclosed medical issues that prevented him getting licensed in California. Before that he was a solid mid-level heavyweight.
They may not win this competition, but they are the fighters you'd least like to meet in a Russian prison.
Big Show Appearances (X2): 40
What can be said about the Diaz boys that hasn't already been covered ad nauseam here and elsewhere around the interwebs? These brothers come to fight and don't particularly care if it's backstage or in the cage.
Big Show Appearances (X2): 32
Years ago, Alistair was Valentijn's younger, skinnier brother.
My, how things have changed.
Big Show Appearances (X2): 46
As Chael Sonnen likes to point out, Rodrigo survived a collision with a truck when he was a boy. Is it any surprise he's taken the best shots from MMA's toughest heavyweights?
Poor Rogerio. He's beaten legends like Sakuraba, Ortiz and Overeem.
But he'll always play second fiddle to his twin.