How to deal with lying

[Yellowfish2020]

11 year old step daughter just can't stop lying. She's only recently come to live with us full time as her mother is just not able to offer her a safe upbringing (social services and courts all agree). She's great, we have fun, but she lies about everything, even things that are unimportant and clearly won't get her in trouble.
I'm not sure how to handle this, whether we do damage by being too heavy handed, but equally we have tried the softly and understanding approach.
This morning she lied that she had eaten breakfast. Previously it was that she was wearing new trousers not the ones from yesterday and that she didn't know how yesterday's must have got back on the hanger. So it's stupid stuff, inconsequential. And that's what bothers me. She will look you in the eye and say black is white. Over nothing. This is also a trait of her mothers who has real mental health issues.
Her one 'thing' is PlayStation. Her dad is all in favour of taking all screen access from her. At a loss as to what else to do. Have tried to explain and articulate why truth and trusting relationships are important- but it doesn't make a difference. Some of the stuff she's lying about is really odd - saying she is wearing black trousers when they are grey (this is not a vision thing - there is a clear difference) and I just don't know what to do. As a step parent without my o an kids, I'm not sure how unusual this is and how to deal with it.
It does seem 'wilful' so understand the punishment approach - but it's the 'type' of lie that seems more like a much younger child that worries me

Hi @Yellowfish2020

You're doing a great job taking on a child that isn't yours, especially with no first hand experience. Thumbs up to you 👍

In my own experience, one of my dc started to lie a lot, and we would explain that if they kept on lying, we wouldn't know when they were telling the truth or not. It didn't seem to work at first, but when issues came up and they lost out because I couldn't believe what they were saying (and told them so), things changed pretty quick

Really kind of you, thanks for your reply. Really good to hear others have been through it and managed to stop it! I just want to build trusting relationships and it's a tough one! Thanks again

Is it malicious lying? If it's fairly casual I'd just try and make a joke of it that gently says 'I realise this is bullshit and I'm gonna call you out for it but I support you and am not going to shame you' kindof way if that makes sense. If you rise to it or punish it she's only getting the validation she needs even if it's in a negative way IYSWIM. I can't imagine a break off from mum has been easy on her whatever the circumstances. I would look up therapeautic parenting and PACE on youtube. These will give you lots of ideas around positive parenting and I think this is ultimately what she'll need x x

If a child has suffered trauma of some
Sort... they often develop habitual
Lying even over trivial things which is frustrating.
Has she had any support or 1-1 from an external agency?

It sounds like she's had/is having a tough time. I would try and get some support for her/someone she can talk to outside of the family.

I'd ignore all the small lies, it just doesn't really matter. If it's a big or malicious lie I'd sit down and talk to her about trust and why the truth is important. Given the difficult time she's had I would try and avoid punishment at all costs. When she tells the truth about something I would really praise her up. Draw attention when she tells the truth, even if it's something really small and try and reward her. Show her the truth is important through positive reinforcement rather than negative.