I used to get that *all the time* for as long as I can remember. Ever since I met my husband I've been smiling a lot more though... there's a difference between "perfectly well" and "really happy", I guess.
It's kinda weird, because I still struggle with depression and dysthymia quite regularly, but my husband just has this ability to make me smile or laugh several times a day anyway... I still rarely smile at other people though.
Laughing... 'heeheehee' is what I usually do nowadays... for a long time though my laughs were indistinguishable from dolphin squeaks though... occasionally still are. Sometimes I laugh in the more "normal" way people laugh in. I like my dolphin squeak laugh btw, other people mostly don't. <grin> er <squeak> Volume varies.
I'm the older sibling as well (although not a brother), and though the above is true, I don't think laughing is something siblings tend to teach each other... but then and again, I could be wrong. Iirc my (little) brother laughs more normally than I do though...
And my brother told me, "That's not actually laughing, Mandy."
Then he told me that to laugh you:
1. Don't try to make noise while inhaling.
2. Do a lot of it under your breath (and he demonstrated).
3. Do only a bit of it out loud (he also demonstrated).
And then he demonstrated what laughing actually sounds like and got me to imitate it until I got it right.
Okay... I'm pretty darn sure that most kids can laugh normally by that age, but you could go hang out near a kindergarten and observe the kids to make sure. 
The dolphin sound I mentioned earlier is what happens if I laugh so much that I get out of breath and I start making noise on inhaling instead of exhaling, or when I really want to laugh but try to suppress it (again, noise on inhaling instead of exhaling). But I find it so amusing to listen to my own dolphin laugh... 
When I first learned to whistle I did it the wrong way too... I can whistle by going "sssssssss" and it actually does sound like a sort of whistling. I learned to whistle properly a few years later, but I can get a higher volume with the "ssss"-whistles.
Why? Anyway, considering that most people have like 2 kids nowadays, about half the people should be the eldest sibling...
This reminds me of an interesting article that relates birth order to level of success. It's been a while, but I found it. In fact, my search turned up lots of info on the subject.
http://www.discovervancouver.com/forum/t...C_ID=84671
I wonder what role ASD's might have in such statistics if considered.
more of a
HA! HA! HA!
Usually my laughing at something makes other people laugh as well, even if the thing I'm laughing at isn't particularly funny.
For instance, my part time job (well full time through holidays) involves dealing with emergency calls. We have our regular batch of mentally ill people who phone in - and I don't mean suicidal folk, I mean people who phone in saying they know who did 9/11, where Madeline McCann is, their houses are being watched etc. Some of them do phone in from psychiatric wards, but others sit phoning from their homes.
Anyway we have this one woman called Helen who phones in all the time and always says "hello there mrs police officer layyydeeeeee" and wants to talk about her paranoia/hallucinations etc. Most of the time she's in a good mood but other times she's really depressed. Anyway. She phoned in one night and got one of my mates at work, because people were stealing her gnomes. Conversation (which still cracks me up typing it) goes like this:
Helen: "they've been stealing my gnomes!!!!! They're heirloom gnomes, my mum gave me them and they've got authentic German paint on them. Do you know what I mean?"
Op: "yes Helen, they're very nice."
H: "You don't know what I mean - you're lying"
O: "Well no Helen, I don't have gnomes."
H: "Neither do I because they stole them!!!!!!!!!"
I don't know why but the conversation had me in hysterics for about 10 minutes and still makes me laugh.
(Incidentally, before anyone accuses me of mocking mental illness etc - my dad's schizophrenic and I'm that used to hearing all number of 'strange' things all my life that the best coping mechanism I gained was seeing the funny side in it.)
PS I'm an only child.
When my husband laughs, it is a "normal" laugh, unless he really gets going. Then it is an uncontrollable fit in which he will fall out of chairs, fall backwards in chairs, scream with laughter, stop breathing, and then, yes, about 10 minutes will do it again, as it is still funny. People stop and stare, and wonder...
I'd be mad if someone was nicking my gnomes too tbh - I don't actually have any gnomes because I don't have a garden
else I would most likely have a gnome called Spud.
Thats actually quite scary, that sounds like how I laugh when I get going - really manic and totally unable to control myself... I'll end up curled up on the floor shaking.

If something really makes me laugh hard I barely make a sound. Laughing can make me horny. Anyone else have this...ummm problem?