Unfortunately, Fischer suffered from some mental health problems later on in life. They were probably both a little nervous, but we don't know for sure. Anyway, whether someone was afraid or nervous or had mental health issues is really not relevant to who was better at chess. I might be afraid of getting into a fist fight with some guy...that doesn't mean I'm not a better fighter. Same hold true in any sport.
Cowardly Fischer

Check the topic above and you will understand. Fischer was afraid of Karpov because he was much better player.
Why would Fischer be afraid of Karpov, if Fischer was the much better player? ;)

I often see people say Fischer was scared of losing his title to Karpov. From what facts do such accusitions come?
He refused to play then continued calling himself the world chess champion. There were other efforts to get him to play Karpov and chess in general but he always refused. Of course we can only guess at his motivation, but there are only so many choices... it certainly seems very timid.

I think he was more afraid of losing in general. From what I can tell, based on documentaries about him, he really thought he was the greatest ever, and he couldn't live with the possiblility he wasn't. Losing to Karpov would have been a blow he wouldn't have been able to take.
Its also pretty clear his mental health was slipping at this point. Such a shame too it would have been a great match.

Fischer was a chess genius. He was also profoundly disturbed. Let us thank him for his contributions to the game and then let him rest in peace.

Fischer was a chess genius. He was also profoundly disturbed. Let us thank him for his contributions to the game and then let him rest in peace.
I agree.
"I often see people say Fischer was scared of losing his title to Karpov. From what facts do such accusitions come?"
From the fact that he refused to play the title match against Karpov.

I attended one of George Koltanowski's "Knight's Tour" demonstrations in the late 70's. Koltanowski was involved in the negotiations for the Fischer-Karpov match.
According to Koltanowski, Fischer would make a demand, it would be met. Fischer would make another demand, it would be met, etc. Eventually, everyone involved, grew weary of Fischer's continual demands, and the match was cancelled.
For whatever reason, it looked as if Fischer did not want to play Karpov.
Fischer was not afraid of Karpov.
"Fischer was scheduled to defend his title in 1975 against Anatoly Karpov, who emerged as his challenger.[417] Fischer, who had played no competitive games since his World Championship match with Spassky, laid out a proposal for the match in September 1973, in consultation with FIDE official Fred Cramer. He made three principal (non-negotiable) demands:"
- The match continues until one player wins 10 games, draws not counting.
- No limit to the total number of games played.
- In case of a 9–9 score, the champion (Fischer) retains the title, and the prize fund is split equally
These were pretty standard terms from previous championships. FIDE refused, so he said fine, keep your silly Championship title, I quit.

Prudentia
Could be. It's unfortunate that he quit playing. He generated a lot of interest in chess in the US, even amongst non-players.

It is very unfortunate that he quit playing indeed. However, it makes one wonder if he'd still be the legend that he is today had he played Karpov and lost.. Tupac Shakur once said, "mystery sells." ;)

Chessgames.com has a brief but worthwhile overview of the failed negotiations.

I think that he might still be a legend among older US players. I still remember the publicity about this 10 or 12 year old who was beating the top players at the Manhattan Chess Club.

Fischer was not afraid of Karpov.
"Fischer was scheduled to defend his title in 1975 against Anatoly Karpov, who emerged as his challenger.[417] Fischer, who had played no competitive games since his World Championship match with Spassky, laid out a proposal for the match in September 1973, in consultation with FIDE official Fred Cramer. He made three principal (non-negotiable) demands:"
The match continues until one player wins 10 games, draws not counting. No limit to the total number of games played. In case of a 9–9 score, the champion (Fischer) retains the title, and the prize fund is split equallyThese were pretty standard terms from previous championships. FIDE refused, so he said fine, keep your silly Championship title, I quit.
Never saw his conditions laid out like this ... why the heck did they decline them again? It all sounds fair and straightforward to me.

Fischer also wanted the match to be played in private. Aside from an adjucator, I don't think he wanted anybody else around. No cameras, no live feeds, nothing. I think that was a major reason why the match fell through.

Fischer was not afraid of Karpov.
"Fischer was scheduled to defend his title in 1975 against Anatoly Karpov, who emerged as his challenger.[417] Fischer, who had played no competitive games since his World Championship match with Spassky, laid out a proposal for the match in September 1973, in consultation with FIDE official Fred Cramer. He made three principal (non-negotiable) demands:"
The match continues until one player wins 10 games, draws not counting. No limit to the total number of games played. In case of a 9–9 score, the champion (Fischer) retains the title, and the prize fund is split equallyThese were pretty standard terms from previous championships. FIDE refused, so he said fine, keep your silly Championship title, I quit.
Never saw his conditions laid out like this ... why the heck did they decline them again? It all sounds fair and straightforward to me.
9-9 tie going to Fischer is not fair, as then Fischer only needs to win 9 while Karpov needs to win 10, so Karpov starts the match a game behind.

Chessgames.com has a brief but worthwhile overview of the failed negotiations.
I wish I had been able to record Koltanowski's comments. He provided some details on the behind-the-scene negotiations. But, after almost 40 years, it's been forgotten.

"I often see people say Fischer was scared of losing his title to Karpov. From what facts do such accusitions come?"
From the fact that he refused to play the title match against Karpov.
The idea that Fischer was afraid of anybody over the chess board is a preposterous idea. Was he scared to play Larsen or Taimanov when he beat them both 6-0 ? no he was not. Was he scared to play Petrosian when he beat him 5-1 with 3 draws in the candidates final in 1971 ? no he was not . Was he scared to play Spassky when he beat him 12.5 to 8.5 to win the world title in 1972 ? no he was not.
Garry Kasparov said upon news of Fischer's death that Fischer was the most dominant player that the chess world has ever seen. He also said that Fischer was a revolutionary and an innovator who made chess the sport it is today. Nobody had ever seen the sort of chess that Fischer played ..fighting to win with the black pieces.
He left the sport at the top on his own terms..for anyone to suggest that Fischer was afraid of Karpov knows nothing about Bobby Fischer and the idea that he didn't play Karpov so he must have been scared of him is frankly a non-sequitur.
I often see people say Fischer was scared of losing his title to Karpov. From what facts do such accusitions come?