What it means
When a lender writes hundreds of mortgages, it can bundle them into a trust and sell bonds backed by the monthly repayments. If a government-linked agency wraps a guarantee around those bonds, the result is an agency MBS.
If no agency is involved, it is a non-agency MBS, and every dollar of credit risk sits with the bondholders. The distinction matters because it changes what an investor is actually buying.
An agency bond is mostly a bet on interest rates and how quickly borrowers repay early, while a non-agency bond is also a bet on whether those borrowers keep paying at all. Loans end up in non-agency deals for two main reasons: they are too large to qualify for agency purchase, or the borrower does not fit standard documentation rules.
That second group includes self-employed people, investors buying rental property and borrowers with a recent credit blemish. Both groups can be perfectly sound credits; they simply sit outside the agency rulebook.
To make the senior bonds attractive to conservative buyers, the deal is sliced into tranches that absorb losses in a set order. The bottom tranches take the first losses and pay the highest coupons, while the top tranche only suffers once everything below it has been exhausted.
The size of that cushion, called subordination, is the single most important number in the structure. Non-agency issuance shrank dramatically after the 2008 housing downturn and returned in a more conservative form, with tighter underwriting and larger cushions.
Anyone reading a modern deal should still look past the credit rating to the underlying loan characteristics: average loan size, how much equity borrowers hold and how concentrated the pool is by region.
In practice
Real-world examples.
Example
A regional bank has written $600,000,000 of mortgages that are too large for agency sale. Rather than keep them on its balance sheet and tie up capital, it securitises them into a non-agency deal and sells the senior bonds to insurers. The bank retains the riskiest 5% slice so investors know it has money at stake.
Example
A pension fund manager is offered a non-agency bond yielding 1.4 percentage points more than a comparable agency bond. She models a severe downturn, finds that losses would need to exceed 14% of the pool before her tranche is touched, and buys a $20,000,000 position.
Example
A treasury team at a mid-sized insurer reviews its holdings and discovers that 40% of its non-agency exposure sits in one coastal state. It sells part of the position and reinvests in a deal with borrowers spread across twelve states, reducing the risk that a single regional shock hurts every loan at once.
Think of it
“Non-agency MBS is mortgage bonds without government backing-private label securities.
Formula
Calculation
Subordination (%) = Total subordinate tranches / Total collateral pool x 100
A private lender pools $500,000,000 of large residential mortgages. It issues a senior tranche of $425,000,000 and subordinate tranches totalling $75,000,000. Subordination supporting the senior bond is $75,000,000 / $500,000,000 = 15%.
Suppose the pool eventually loses $60,000,000 to defaults, which is 12% of the original balance. The subordinate tranches absorb the entire amount and are left with $75,000,000 - $60,000,000 = $15,000,000 outstanding, so the senior bond is repaid in full.
If losses instead reached $80,000,000, or 16% of the pool, the subordinate tranches would be wiped out and the senior bond would absorb the remaining $5,000,000. That is a loss of $5,000,000 / $425,000,000 = 1.2% of the senior balance.Case study
Seen in the real world.
Harbourline Residential Capital is an illustrative, entirely fictional specialist lender that funds mortgages for self-employed borrowers. Because these loans are documented with two years of business bank statements rather than payslips, no agency will buy them, so Harbourline must either hold them or securitise them privately.
In this illustrative scenario the firm pools $350,000,000 of such loans and issues $290,500,000 of senior bonds, leaving $59,500,000 of subordinate tranches, or 17% subordination. Investors initially push back, noting that the borrowers are harder to assess, so Harbourline agrees to retain the bottom $17,500,000 slice for the life of the deal.
Three years later, a regional employment shock pushes cumulative losses to 6% of the pool, or $21,000,000. The subordinate holders take real pain and Harbourline's retained slice is largely gone, but the senior bonds continue paying on time. The illustrative lesson is that subordination is not a formality; it is the number that decides who loses sleep.
Watch out
Common mistakes.
- Assuming a AAA rating on a non-agency bond means the same thing as a government guarantee. A rating is an opinion about the odds of loss, not a promise that someone else will pay.
- Treating all non-agency deals as subprime. Many pools contain high-quality borrowers whose loans are simply too large or too unusual for agency purchase.
- Focusing only on yield and ignoring subordination. Two bonds can offer similar coupons while sitting behind very different loss cushions.
Questions
People also ask.
What is the practical difference between agency and non-agency MBS?
Agency bonds shift credit risk to the guaranteeing agency, so investors mainly face interest rate and prepayment risk, while non-agency investors face borrower default risk as well.
Why do investors accept that extra risk?
Because they are paid a wider yield spread, and because well-structured senior tranches can survive loss levels far above anything typically experienced.
Can a company issue a non-agency MBS backed by commercial property?
Yes, although a pool of office, retail or industrial loans is normally described as a commercial mortgage-backed security rather than a residential one.
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