Perfect testicular symmetry is rarer than most men assume

No. The normal volume differential and the left-lower suspension pattern have no association with reduced fertility or sperm quality. If fertility is the worry, a semen analysis is the test that answers it, and how the two sides look or feel tells you nothing on that front. The wider male numbers, from refractory periods to premature ejaculation, are in our sex statistics roundup.

The right, by a small margin, at 9.69 ml against 9.10 ml on average in Chang et al.'s post-mortem measurements, even though the left usually hangs lower. Size and suspension height are two separate axes and they don't have to line up. The commonest pattern is a lower-hanging left that's smaller than the right, and every other combination occurs too. Both axes are normal on their own.

A varicocele is a swelling of the veins inside the scrotum (the pampiniform plexus), much like varicose veins elsewhere, and it sits on the left in approximately 85 to 90% of cases for the same reason the left hangs lower, the perpendicular drainage of the left testicular vein into the left renal vein and the backpressure that creates. Around 15% of men have one, often without symptoms. Varicoceles turn up in 35 to 40% of men assessed for infertility, though whether they cause it is still argued over. The affected side often feels warmer and may carry a mass you can see or feel, usually described as a bag of worms. Any new swelling should be checked clinically.

It's the most common cancer in men aged 15 to 35, with about 2,320 new cases a year in the UK and an estimated 9,810 in the US in 2026, and it's also among the most treatable, with five-year survival of about 99% when localised and 72% even once it has spread to distant organs. The signs to act on are a new lump or swelling on one testicle (usually painless), heaviness in the scrotum, a dull ache in the lower abdomen or groin, or a change in size or texture. Any of those means a GP appointment. Regular self-examination is how most cases get found early.

Yes, through well-mapped mechanisms. Sperm production needs approximately 2 to 4 degrees Celsius below core body temperature, and even modest sustained warming cuts sperm production and motility. If you're trying to conceive, the practical steps are looser underwear (boxer briefs keep scrotal temperature lower than briefs), no long spells with a laptop on your lap (sustained studies show a 2 to 3 degree rise), no long hot baths or saunas, and no excessive cycling. The effect reverses. Sperm quality usually recovers within 74 days, one full spermatogenesis cycle, once the heat source is gone, so it matters for fertility planning and hardly at all for day-to-day health.

Yes. Trauma can cause a haematocele (blood collecting in the scrotum), orchitis (inflammation) or, at worst, testicular torsion, a twisting of the spermatic cord that's a medical emergency needing surgery within hours to save the testicle's function. Serious injury or torsion can be followed by post-traumatic atrophy, which leaves a permanent size difference, and epididymo-orchitis (inflammation of testicle and epididymis, usually from infection) can swell one side temporarily and then partly shrink it. Any sudden change in size after an injury needs urgent assessment, because telling traumatic swelling apart from torsion takes a clinician.

A moderate positive correlation across a population, and a poor predictor for any one man. Testosterone comes from Leydig cells, which make up about 10 to 15% of testicular volume by mass, so total volume tracks sperm-producing capacity far more closely than hormone output. Low testosterone (hypogonadism) can't be read from how the testes look or feel, and a blood test is the right check, which our testosterone percentile calculator reads by age. Ordinary size variation says nothing about it.

From the start of puberty, around age 13 to 15, according to most urology and oncology guidelines, because testicular cancer incidence climbs steeply through adolescence. The standard advice is once a month after a warm shower, when the scrotum is most relaxed, rolling each testicle gently between thumb and fingers to feel for lumps or a change in consistency or size. Most boys are never taught this. Knowing what's normal for you is what makes a change easy to spot, and the habit is worth keeping through your 30s and 40s.

With which one hangs higher, yes, and with size, apparently not. Chang et al. (1960) found the right side sat higher in 64.7% of right-handed men, which is the ordinary left-lower arrangement, while 68% of left-handed men had the left side higher. Bogaert's 1997 analysis of Kinsey Institute interviews found the same leftward pattern, weaker in men who weren't right-handed. Chang's post-mortem measurements found the right testis usually the larger, but didn't tie that to handedness.

The proportions mostly hold while the absolute sizes move. Testes grow fast during puberty, gaining most of their volume between ages 11 and 17, and after peak reproductive age they shrink gradually from the 40s onward as Leydig and Sertoli cell populations fall. The asymmetry is set early and stays roughly the same in proportional terms through adulthood even as both sides change. A sudden shift in asymmetry at any age, as opposed to slow age-related change, is the sign that needs looking at.

Orchitis is inflammation of one or both testes, usually from bacterial infection (often STIs such as chlamydia or gonorrhoea in younger men, E. coli and Klebsiella in older men) or from a virus, with mumps orchitis affecting approximately 30% of post-pubertal males who catch mumps. It arrives fast, with rapid swelling, real pain, warmth and often fever, which is nothing like the slow, stable, painless asymmetry of ordinary anatomy. Bacterial cases get antibiotics and viral ones supportive care. Mumps orchitis leaves permanent atrophy in 30 to 50% of affected testes, one reason MMR coverage matters for male reproductive health.