A global meta-analysis estimated a 51.6% decline in mean sperm concentration among men unselected by fertility between 1973 and 2018. That deserves attention. This population trend does not identify one single cause. Read the analysis.
In a study of 29 men, an hour with a laptop on the lap raised scrotal temperature by 2.6–2.8°C. The same seated posture without a laptop raised it by 2.1°C. Both the posture and the device mattered. The study measured temperature, not changes in semen or fertility.
Measured scrotal temperature rise in a 29-man study.
Same posture without a laptop: +2.1°C. This experiment did not test hot seats or sperm counts.
Zero sperm in semen in a small 14-man study using a polyester scrotal suspensory sling, day and night for 12 months. Counts recovered after removal. Not an ordinary-boxer or NUMBNUTS trial.
Shafik’s 1992 human experiment used a polyester scrotal suspensory sling in 14 men. It did not compare normal boxers, cotton or wool. A separate 1993 experiment used 24 dogs in cotton or polyester garments, plus 7 controls, and found adverse changes in the polyester group. These are different studies, and neither demonstrates that merino underwear improves human fertility.
A systematic review of eight articles found a positive trend with scrotal cooling, but not enough evidence for firm conclusions about fertility. Better controlled trials are needed. These studies did not test NUMBNUTS.
Sperm concentration measures the number of sperm per millilitre of semen.
Motility describes sperm movement, including how many are moving progressively.
Morphology describes the proportion of sperm meeting the study's criteria for normal shape.
Nikolopoulos et al., systematic review (2013), PMID 23654310. The review covers the cooling methods studied, not NUMBNUTS.
100% merino wool through the boxer body and centred sleeve. Natural rubber at the waistband. Two reusable inserts designed to absorb body heat as they soften. A practical cooling routine, without loose ice.
1. Shafik (1992), human polyester-sling experiment, Contraception 45(5):439–451.
2. Shafik (1993), dog textile experiment, Urological Research 21(5):367–370.
3. Sheynkin et al. (2005), laptop use and scrotal temperature, Human Reproduction 20(2):452–455.
4. Levine et al. (2023), global sperm-count meta-analysis, Human Reproduction Update 29(2):157–176.
5. Nikolopoulos et al. (2013), scrotal-cooling systematic review, Journal of Obstetrics and Gynaecology 33(4):338–342.
Natural fibre against your skin. Targeted cooling at the centre.
A synthetic plastic fibre.
100% merino wool in the NUMBNUTS boxer body and centre sleeve.