This.
@Alphavilla
I know many people have said pretty much what I am about to say, but nevertheless... this is my take on it...
Until employers stop being selfish and grabby, and give actual PROPER contracts to people, like at least 24-28 hours, (that will enable people to be able to afford to live without benefits a bit easier - or just have a small top-up,) then no, people will NOT work more hours than 16.
As the poster above said, it will screw with their benefit payments for their rent and council tax. I have seen it happen multiple times to people... If they work a few extra hours in any given week, (even just 4 to 7 hours,) they end up with benefit overpayments, and they end up in a mess financially.
Where my brother works, they have about 12 people who have just 4 and 8 hour contracts, but they do extra hours quite often because the employer needs them to. They are often asked to do more than 16 too (some weeks) but they refuse, and the shift manager ends up doing it. If they offered real, proper decent contracts, people will do the hours.
4 and 8 hour per week contracts are a piece of shit. Even 10 to 12 ones are pretty useless. DH gets fed up of his job sometimes, and has applied some 5 or 6 times in the past few years, to supermarkets and shops and other customer service type roles. ALL the jobs had no more than 10-12 hours guaranteed. No-one can live on that amount of hours!
Yeah, they will offer extra hours sometimes, but they don't HAVE to. And they sometimes don't. Indeed, I have seen 'salaried managers' work extra hours cleaning and serving the public themselves to save on labour, and give the '4-8 hour contract' people JUST the 4-8 hours. And from mid January through to mid March, when custom is a bit low, the employees get dropped to 4-8 hours a week again.
If employers want people to do more hours, GIVE THEM THE HOURS IN THE FIRST PLACE. Stop with the shitty low hour contracts. Pre 2005-ish, this was not even a thing. When DH applied for the job he's in now (customer service role,) circa 2003, every job he looked at was 18 hours, 24 hours, 32 hours, and 40 hours a week contract. There were no 4-8 hour contracts. They're farcical. As I said, don't offer people wanky hours like this, and then cry off when they won't do lots of extra hours when YOU want them to.