We're all aware of body language, even if most of us aren't experts on it. We know that if someone makes direct eye contact and intrudes on our personal space (most Americans share the Anglo-Saxon "arm's-length" personal space expectation), they are trying to intimidate us. A lowered head and raised eyes with a slight smile indicate shyness, while an unwillingness to make eye contact means a person is not being honest.

So when a body language expert, in this case, ex-FBI agent Joe Navarro, watches a presidential debate, some interesting observations can arise.

On Biden:

Biden entered the room first, and his stiff walk, with a short, narrow stride, immediately showed his age. Contrary to what they say online, that’s not necessarily a cataclysmic sign of mental decline. But as we age, we lose a lot of the nerves that help us keep our balance. The Merkel, Meissner, Ruffini and Pacinian nerve cells on and just below the skin — all so robust when we are young, able to detect the most delicate touch, the slightest change in pressure and temperature, the subtlest movement or vibration — begin to lose sensation as we age.

And: 

I often say that body language speaks volumes, but Biden’s volume was full of blank pages. Sure, it might be because he wanted to be respectful and listen intently. But it could also be because, as we age, our gestures become more economical. Biden’s nonverbal communication did nothing to counter the narrative that his age is catching up to him.

Interesting; Mr. Navarro does not comment, we must note, on the content of either candidate's statements, just the presentation thereof; but he does let some preference towards Biden slip through. Even so, look at that second clip — "Biden's volume was full of blank pages" — and Navarro implies this is due to Biden wanting to be respectful or listen intently, or that his gestures are becoming "more economical." He leaves out the obvious reason: Joe Biden was lost, cast adrift on the Sea of No Teleprompter, and had very little idea what he was supposed to be doing.

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On Trump:

The first thing I noticed about Trump was his tan. You might think that sounds superficial, but appearances can make a big difference. Just think of how disheveled Richard Nixon looked — with his 5 o’clock shadow, lack of makeup and pale complexion — next to a young, tanned, make-up-wearing senator named John F. Kennedy in the first televised debate in 1960.

In a similar way, Biden looked white as a sheet next to a sun-kissed Trump. Humans naturally intuit health, vitality and energy in a tanned face. It was another indicator that Biden appeared somewhat frail.

I would argue that Biden appeared somewhat frail because Biden is somewhat frail, and it's probably best to leave out the "somewhat."

Several times, when Trump heard something negative, he exhibited a behavior I call a deflective pseudo-smile.

In a true, genuine smile, also called a Duchenne smile, we see an organic movement of the lips and a crinkling of the eyes. But with Trump’s deflective smile, the lips compress tight, the commissures (or corners) of the mouth tightly pull upward and laterally, almost like you’d see on a clown’s face, and the eyebrows arch quite high. This makes the smile look contrived, intended to nonverbally fend off a verbal attack. It’s a way to pretend something wasn’t hurtful when, in fact, it was.

Every human I've ever known does this; I do it myself. Implying it's something in particular to watch for in Donald Trump is a trifle misleading. It's kind of an ingrained response when we detect someone dissembling.

There are some interesting observations here, nonetheless. Humans are social animals, and we communicate not just with our voices but also with our faces — especially with our faces, and humans are somewhat unique among mammals in having very expressive faces. We communicate through our posture, through our stance; we communicate with gestures and head movements. Human communication is very complex, as befits us, as we are behaviorally very complex beings compared to other mammals.

Most of us aren't experts in body language, true. But it doesn't take an expert to detect that after last night's performance, the Biden campaign is in deep doo-doo.