Bats are bugs, y’know?
And not just back then!
Look at the official photos taken of military recruits in their uniform – they are almost never smiling, but showing a serious attitude. (Marines, especially.) Because that’s the ‘fashion’ in the military for those photos.
Same thing with most sports photos. Look at a photo of a high school sports team – unsmiling, serious expression. Also most photos of individual players – posed, with a serious or even aggressive expression on their face. Same thing; that’s the fashion for high school sports photos.
Queen Victoria is said to have had an enormous collection of “Dead Book” photos- photographs of friends, servants, acquaintainces and even some strangers taken post mortem, sometimes in their coffins and sometimes in seated positions or beds.
I was in a small house museum in Georgia once where there was a framed picture of a sleeping baby and the guide even pointed out how peaceful the baby looked. I pionted out- very tactfully- the baby was probably dead. (Very few parents would dress the child in an ornate baby gown and then place her on a table and wait til she’s asleep to take a nap.) The guide, not a professional historian but just a docent, acted as if I’d dropped the F-bomb and said “What a morbid idea! Who would do that?” so I let it go, but the answer is “quite a few people”; infant mortality was high and some people wanted a keepsake.
For the very morbid in nature: Victorian era photo of a father and his dead baby.
That’s still done nowadays when a baby dies at birth.
There are some online archives of Victorian post-mortem photography. I find them very affecting - spent a whole slow day at the desk once looking through them. My boyfriend says I’m morbid.
I have a photo of my mother’s family, taken when she was about 5 . . . that would be about 1917-18. It’s a very formal photo and nobody is smiling. If you look very closely you can see that one of my aunts is crying. The reason is that my grandfather caught her smiling, and he smacked her.
But why would it be any easier to hold the same exact solemn face for the same amount of time?
I guess back then, they couldn’t hold a two fingered gang sign for 10 minutes either 
The remarks referencing death are on point.
Not only were portraits made of people who were in fact dead at the time of the photograph, but formal portraits of healthy living people (something that, for some, happened once in their lifetime) were also understood as a legacy for eternity. They would have seen mirth, or any fleeting emotion, as inappropriate for the same reasons that most modern folks think casually-dressed corpses are weird.
Our understanding of photography is based on it being cheap, fast, and common, and most of our pictures are intended only for the lifetimes of those pictured. None of this was true in the 19th century.
Another reason might be that the meaning of a smile often differs between cultures/countries, and the past famously is another country.
The OP assumes the present-day US semantics of a smile, where smiling is common in baseline not-unpleasant social interactions. (This meaning is probably why US smiles are often misunderstood as ‘fake’ in contemporaneous cultures like Germany where smiling usually means a level of affection or enjoyment well above baseline).
So, might 19th century Americans perhaps just have smiled less overall?
A face with all the muscles completely relaxed looks pretty solemn. There are far fewer muscles you need to activate to look simply not-dead-or-asleep-but-solemn than to look animated-and-happy. For not-asleep-but-solemn, you can get away with as little as opening your eyes and closing your mouth, both relatively stable positions; for smiling, many other muscles are involved, and most of them have many possible degrees of tension in normal use.
Long story short: Really, it’s just easier. Try it.
Try it. Seriously - right now. The boyfriend does wet-plate sometimes as a hobby and the funny thing is that when you hold the face you can hold for sometimes fifteen seconds the photo comes out looking just like a 19th century person - in other words, whenever anybody asks “why did people look different in the past?” it isn’t the standard answers of clothing or teeth - that’s just the way your face looks when you’re staring at a camera for that long. You look like a little drummer boy, scarred by the horrors of war.
(Taking pictures of dead babies) In fact, it’s sometimes done when the baby died soon after birth, as well. Sometimes people want that last picture, especially if it’s the only picture.
Here’s a pic of Queen Victoria smiling :
Daguerreotype Exposure Times
1839 Daguerreotype half-plate & whole plate 15-30 minutes
1841 Daguerreotype ninth-plate & sixth- plate 90 seconds
1842 Daguerreotype ninth-plate & sixth- plate 60 seconds
So the reason they look dour in those 19th century photos is that holding a smile for the entire exposure time would likely appear to be more of a grimace than a smile.
Interestingly, here is the first American Daguerreotype: http://z.about.com/d/inventors/1/0/8/N/daguerreotype.jpg. This was circa 1839. If only the technology had come together a little over 10 years earlier, we could possibly have had a photo of Thomas Jefferson and John Adams, albeit as old men.
And here is one of the Duke of Wellington.
It’s strange to look at an actual photo of a man who fought Napoleon Bonaparte.
I fall into the camp that doesn’t agree whole heartedly that not smiling was easier for long exposures. This just smacks too much of urban legend. Without some better evidence that this is true, I find it to be a apocroyphyl “just so” story.
It makes more sense to me that peaple just “didn’t think” about smiling for photographs. Imagine, you are the first person to be photographed in the world. Would you really think you should smile? We do it all of the time now, because its traditional. Someone had to invent smiling for the camera first…then everyone else probably thought “what a great idea! I never thought of that!”
I wonder what other technological memes started AFTER the technology had been around for a while. “Hello?” for answering the phone? Playing music on the radio? Driving or riding horses on the “correct” side of the road? Right and left footed shoes?
To me it makes perfect sense. Try holding a smile for 90 or more seconds. What smacks of urban legend is the idea that people were grim and unhappy in those days.
Visit Shorpy.com. You’ll find that as technology improved and exposure times were greatly reduced,voila’ , you begin to see people smile. Such as in my above posted pic of Victoria at her diamond jubilee in 1897.
Certainly not after about 1880, with the advent of faster dry gelatin plates and roll film, as I stated above. The OP said “mid 1800’s to at least the 1930’s.”.
I’d split the difference. It WAS an issue during the Daguerreotype / wet plate collodion eras.
If you look around you can find “snapshot” photographs from around the turn of the 20th century or so, where people weren’t posing for the camera. Smaller, lighter cameras such as the early Kodak roll film models allowed it. When deliberately POSING for the camera, however, people seemed to adopt a certain amount of gravitas.
(BTW, the first Kodak roll film cameras weren’t designed to be loaded by the user. You bought the camera with a roll of film in it. You shot the roll, and sent the entire camera back to Kodak. They unloaded it, processed the photos, and sent them back together with the camera, loaded with a fresh roll of film. It was a couple years before they got the idea of putting a paper leader on the roll so the camera could be loaded in daylight by the photographer.)
I’ve mentioned before having read, somewhere, that in “very early times” smiling was a gesture of submission. I haven’t been able to find a citation anywhere to back this up, but it does seem reasonable. If you smile at someone you are not only being friendly and pleasant, but you are also sending a message that you do not intend to threaten or harm the person.
I imagine a lot of people had terrible teeth before 1930 or so; wouldn’t that have been another factor?