Pain fetish (algolagnia)
Algolagnia (pronounced /ælgəˈlægniə/) (from the Greek άλγος, algos, "pain", and λαγνεία, lagnia, "lust") is a sexual tendency which is defined by deriving sexual pleasure and stimulation from physical pain, particularly involving an erogenous zone.
Studies conducted indicate differences in how the brains of those with algolagnia interpret nerve input.
History of Research
Havelock Ellis was one of the first researchers to look into algolagnia, in the early 1900s. With such titles as Analysis of the Sexual Impulse, Love and Pain, The Sexual Impulse in Women and The Evolution of Modesty, The Phenomena of Sexual Periodicity, Auto-Erotism, he described the basics of the condition. Eugen Kahn, Smith Ely Jelliffe, William Alanson White, and Hugh Northcote were other early psychological researchers into algolagnia.
Most early research placed algolagnia in the same category as masochistic tendencies and other "deviancies of sexuality", but William Alanson White believed that algolagnia was primarily a motor function or nerve disease.
Currently, most psychological researchers feel that algolagnia is a physical phenomenon in which the brain interprets pain signals as pleasurable leading to psychological effects.  In particular, the research conducted by Dolf Zillmann indicated that:
...most algolagniacs see their actions as an active lust, not a motivational one. Patients with algolagnia could lead normal lives, enjoy normal arousal sequences, and indulge in fairly normal sexual intercourse, but when exposed to sexual pain, were unable to control their reaction. One woman described it as being unable to prevent her arousal or subsequent orgasm due to pain, even if she was not aroused when it began. 
This, along with other research , often links algolagnia to aggression, or hypersexuality, or other control psychoses.
More recently, modern researchers using MRI and computer models of neuron firing patterns show that most algolagniacs experience pain differently from others. Algolagniacs may have DNA errors such as SCN9A, causing inaccurate nociception to occur. 
At least one researcher in the 1900's  thought that algolagnia was a psychological disorder. Dr. Schrenck-Notzin's assertion of this contributed largely to the blurring of the lines between algolagnia and such true psychological conditions as masochistic tendencies and the like. The result is that very little ongoing research has been conducted, with most nerve researchers focusing on nerve diseases and most physioneurologists concentrating on neuropathological reasons for such reactions. 
Algolagnia and Paraphilia
Algolagnia is not a paraphilia as defined by conventional medical literature. While many people, especially those unfamiliar with paraphilias (specifically, physical masochism) and algolagnia, tend to conflate the two as equivalent, this is in error. Paraphilias by definition include mental urges where the activity must be the sole means of sexual gratification for a period of six (6) months. Whereas in algolagnia typical pleasure responses are still sought while seeking pain for pleasure might be avoided or be an addiction. In the case of masochism there is a psychological desire for pain and humiliation which may result in sexual arousal. On the other hand algolagnia is a physiological reaction to pain that doesn't involve desire or psychological aspects directly. In other words a person with algolagnia enjoys the pain but might wish they didn't, and a masochist wishes for the pain but might not actually be wired to enjoy it.
1. ^ a b Kelley, Kathryn; Donn Byrne. Alternative Approaches to the Study of Sexual Behavior, 13-38. ISBN 0898596777.
2. ^ Studies in the Psychology of Sex, Volume 3 by Havelock Ellis - Project Gutenberg
3. ^ Sullivan, Harry Stack (1992). Clinical Studies in Psychiatry. W. W. Norton & Company. ISBN 0393006883.
4. ^ Zillmann, Dolf (1998). Connections Between Sexuality and Aggression. Lawrence Erlbaum Associates. ISBN 080581907X.
5. ^ Bancroft, John D. (1989). Human Sexuality and Its Problems. Elsevier Health Sciences. ISBN 0443034559.
6. ^ Bloch, Iwan (2006). The Sexual Life of Our Time: In Its Relations to Modern Civilization. Kessinger Publishing. ISBN 1428615431.
7. ^ Von Schrenck -Notzin, A (2006). Therapeutic Suggestion in Psychopathia Sexualis with Special Reference. Kessinger Publishing. ISBN 1428625445.
8. ^ Pg 197,Therapeutic Suggestion in Psychopathia Sexualis with Special Reference