The Mossman–Pacey paradox is an

evolutionary Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes, which are passed on from parent to ...

irony Irony (), in its broadest sense, is the juxtaposition of what on the surface appears to be the case and what is actually the case or to be expected; it is an important rhetorical device and literary technique. Irony can be categorized into ...

in which certain actions often taken by men to improve their

sexual attractiveness Sexual attraction is attraction on the basis of sexual desire or the quality of arousing such interest. Sexual attractiveness or sex appeal is an individual's ability to attract other people sexually, and is a factor in sexual selection or ma ...

end up lowering their

fertility Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Fertili ...

. One of the most prevalent manifestations of this phenomenon is the use of

anabolic steroids Anabolic steroids, also known more properly as anabolic–androgenic steroids (AAS), are steroidal androgens that include natural androgens like testosterone as well as synthetic androgens that are structurally related and have similar effects t ...

by some men (particularly if they go to the

gym A gymnasium, also known as a gym, is an indoor location for athletics. The word is derived from the ancient Greek term " gymnasium". They are commonly found in athletic and fitness centres, and as activity and learning spaces in educational ins ...

). These steroids can help build larger

muscles Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...

, but can also lead to side effects such as smaller

testicles A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...

erectile dysfunction Erectile dysfunction (ED), also called impotence, is the type of sexual dysfunction in which the penis fails to become or stay erect during sexual activity. It is the most common sexual problem in men.Cunningham GR, Rosen RC. Overview of male ...

, and lower

sperm count A semen analysis (plural: semen analyses), also called seminogram or spermiogram, evaluates certain characteristics of a male's semen and the sperm contained therein. It is done to help evaluate male fertility, whether for those seeking pregnancy ...

. One of the main reasons for this is that anabolic steroids can stop the

pituitary gland In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a chickpea and weighing, on average, in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The ...

producing the

hormones A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and beh ...

LH and FSH. It has been estimated that approximately 90% of anabolic steroid users are at risk of

infertility Infertility is the inability of a person, animal or plant to reproduce by natural means. It is usually not the natural state of a healthy adult, except notably among certain eusocial species (mostly haplodiploid insects). It is the normal state ...

. Another major example is that the use of

balding Hair loss, also known as alopecia or baldness, refers to a loss of hair from part of the head or body. Typically at least the head is involved. The severity of hair loss can vary from a small area to the entire body. Inflammation or scarrin ...

medications such as

finasteride Finasteride, sold under the brand names Proscar and Propecia among others, is a medication used to treat hair loss and benign prostatic hyperplasia (BPH) in men. It can also be used to treat excessive hair growth in women and as a part of horm ...

can reduce fertility.

Etymology

The term is named after Dr James Mossman and Professor Allan Pacey, who first discovered the phenomenon when they noticed a large number of muscular men needing fertility tests.

References

Health paradoxes Men's health Infertility {{human-repro-stub