There is an association between testosterone levels and step count ... If you walk more, you will see higher levels of testosterone.

There's this concept of too much sleep and not enough sleep. So, the ideal... is 7 to 9 hours. And for men that are not getting that, semen quality tends to be lower. And then for men getting too much, we also see a decline.

As testosterone's produced, it's aromatized in peripheral tissue—you know, fatty tissue. That has a lot of this aromatase, and that converts testosterone to estrogen, so it necessarily lowers the testosterone level that's circulating in our body.

Hot tub/saunas—try and avoid those. There's some light data on seat warmers ... The testicles are outside the body because they need to be a little cooler, so anything that warms them up can certainly be a problem.

One of the sort of landmark studies was about 1,200 men, and it found that men that use cannabis daily had significantly lower [sperm] concentration, motility, morphology compared to those that didn't use it.

When you look at the reasons for infertility, the man contributes probably half of the time to infertility.

In terms of production and heat effects, sperm production is much more sensitive than testosterone production.

There have been some studies that say that maybe DNA fragmentation of sperm can go up a little bit if there's close proximity to a cell phone.

Usually, testosterone peaks kind of early 20s, and it tends to go down probably 1% a year forever.

Men with lower testosterone levels—higher risk of heart disease, diabetes, mortality.

Being more active, I think, is very important. That's another thing that everybody can do to try and improve testicular function broadly but testosterone specifically.

If you look at studies, men with kids tend to live longer.

Over the last 40 years, the youngest father was 11 and the oldest was 88...

There's like these neuropsychiatric conditions you're talking about—autism is certainly one. Bipolar schizophrenia, people have looked at and also link that with older age. Less attainment in school. Failing grades. All that has been shown to be a little bit more common with older fathers.

It's estimated that every year we generate about 2 mutations in our sperm DNA. So, you can imagine that a 40-year-old is going to have, you know, 20 or 40 more mutations than a 20-year-old.

There's even there's one study that I saw that showed that if you look at MRIs of brains of children just after birth, they're actually a little smaller for older fathers compared to younger fathers.

If you look at men over the age of 40, over half will have some trouble with erections. Under age of 40 is probably 15-20%. So, this is a very common condition that we see.

I always tell men, as long as you have a penis, we can always make it hard.

Fertility in itself is a team sport, so it's hard to label a man as fertile or not fertile without knowing about his partner.

There's variations in semen quality around the country and around the world. There's geographic variation.

We know that, as women age, fertility declines; but the oldest father is 96, so the biologic potential certainly persists.