Now that I'm older, it's easier to see some of the inequities in decisions I made earlier in my life. Social media probably calls these things out more openly now than it used to.

We always had joint bank accounts, but I earned significantly more. In my mind I rationalised it: I made more money and had the more professional job, so I should be the one driving the more expensive car. Looking back, I think I had it backwards. The person driving the kids around was the one who needed the comfortable car. That's an easier call to see clearly now than it was then.

What I notice now is a newer version of the same problem. More couples keep their own separate accounts, and there's a lot that's genuinely good in that — personal control, room to make your own choices, dignity over your own spending. A common way to make it work is to take the shared costs and split them down the middle.

What that misses is everything that happens after the shared bills are paid. One person can upgrade the car, buy a new computer, put money away. The other earns less, so those things don't happen. Their car gets older. Their savings stay thin. And slowly the independence the whole arrangement was meant to protect starts to slip — for one of them and not the other.

I think there's a lot to be said for splitting things this way. But it asks a couple to talk about something many relationships haven't learned to talk about yet.

Put a number on it and the gap gets easier to see.

A shared cost of NZ$300 arrives. Split it evenly and the answer looks clean: NZ$150 each. But say one person takes home NZ$900 a week and the other NZ$500. That same NZ$150 is about 17% of one pay packet and 30% of the other.

The dollars are equal. The pressure may not be.

None of this makes an even split wrong. It just shows that the arithmetic can't tell you what fair means for the two people living it.

Couples usually land on a split by habit. Half each feels neutral. Or contributions track income. Or everything goes into one account — or everything stays separate except the bills. Most of these can work.

The trouble starts when the numbers are visible but the reason behind them never gets said out loud.

Because fair is doing a lot of quiet work in that sentence. Does fair mean you each pay the same amount? The same share of what you earn? That you both keep enough personal money after the shared commitments? Does it count unpaid care, irregular work, the cost of a disability, children, one person's debt?

No formula settles that from the outside. And leaving it unspoken doesn't keep things neutral — it just lets whatever you set up first go on making the decision for you.

An even split is easy to understand and easy to run, and it can feel fair when two incomes and two situations are reasonably close.

Splitting by income recognises that the same bill presses harder on the smaller wage. But it raises its own questions: which income figure do you use, how do you handle work that comes and goes, and when does a change in pay change the split?

Keeping a protected personal amount for each of you supports independence and self-respect. It also needs trust, and it should never turn into a licence to audit each other's private choices.

You're not trying to find the one correct system. You're trying to build one the two of you can explain, question and change when life does.

Choose one shared cost: rent, groceries, power, childcare, transport or something else you both recognise as shared. Write down the current contribution and ask:

1. How did we choose this split?
2. What did we mean by fair at the time?
3. Has anything important changed?
4. What principle do we want to use for the next month?

You don't have to redesign every account in one sitting. One cost is enough to show whether the current arrangement still reflects the life around it.

The Money You Both Spend Twice asks what the next piece of shared money should do. This is the question just before it: how does money enter the shared plan in the first place?

Income changes. Care responsibilities move. Health, work, children, debt, housing and personal goals all alter what a contribution means.

An arrangement that was thoughtful two years ago can become uncomfortable without either person doing anything wrong. A review date can matter as much as the original split. "Let's try this for a month and look again" is often more useful than pretending one conversation settles the future.

The Art of the Budget describes budgeting as consultative when other people are involved: what can each person tolerate, and what are you trying to build together?

If money is tied to control, fear, restricted access, coercion or financial abuse, a shared worksheet isn't a sufficient or necessarily safe response. Prioritise safety and appropriate specialist support; TPC's Financial Hardship page may be one starting point.

For an ordinary shared-cost conversation, don't start with the percentage. Start by agreeing what you want the percentage to make fair.