The tax and benefit system is basically broken for couples with children

If you wanted to disincentivise work for the poorest, this is how you'd do it.

Raphaelle and Fabienne pose with one of their three children. Photograph: Getty Images

Last week, this IFS chart showing the effect of the child benefit withdrawal on marginal tax rates made quite an effect:

IFS Chart

I wrote about it myself, arguing that it underemphasised how bad the effect would be on families with lots of children. But the chart also plays down how bad the existing system is. The interplay between benefits, income tax and national insurance results in marginal tax rates which increase and decrease in a haphazard and unpredictable way, and frequently reach astronomic levels.

The IEA's Philip Booth looked at what he calls "the simplest possible case… of a couple with three children not claiming Housing Benefit, Council Tax Benefit or assistance with child-care costs (or a host of other ad hoc benefits that are available)." The figures were compiled in May last year, but are based on the state of affairs this May – so there may be some lags in exact amounts, but the overall picture remains accurate, at least pre-Universal Credit. Here they are charted in the same format as the IFS'

Chart Two

It's clear that for this case, the tax and benefit system is almost entirely backwards. The largest rates are paid at the bottom end of the income scale, progressively declining to 42 per cent, before finally, at the very top end, increasing to a "high" of 47 per cent.

And as Booth says, this is the simplest possible case. For someone in this situation claiming all the benefits they were entitled to, it's highly likely that at least one of the marginal rates would break 100 per cent – and that the tiny patch of 0 per cent at the very poorest end would disappear entirely.

While Bond and I agree on the problem, it's unlikely we would match minds on the solution. To me, the data above illustrates the fundamental folly of thinking that withdrawing benefits can ever really make economic sense. It is, prima facie, a strong argument for a Green-style citizen's income. Remove the conditionality from benefits and you have a system which encourages work at all income levels, rather than stamping on motivation the minute a family is no longer nominally "poor".