Generation-Skipping Transfer Tax — modeling the GST exemption allocation that determines dynastic outcomes

The Generation-Skipping Transfer Tax (GST) was enacted in 1976 and substantially revised in 1986 to ensure that wealthy families couldn't perpetually skip the gift and estate taxes by transferring wealth in trust across multiple generations without paying transfer taxes at each generation. Today, the GST is a 40% federal tax on transfers to "skip persons" — beneficiaries two or more generations below the donor — that exceed the donor's GST exemption.

The GST exemption is the same amount as the unified gift / estate tax exemption — $13.99 million per individual for 2025 ($27.98 million for a married couple). Allocation of the exemption to specific transfers determines whether those transfers (and their future appreciation) are forever GST-exempt or are subject to 40% tax at the skip-generation event.

The structural feature that makes GST planning consequential: the allocation decision is locked at the time of the transfer or the time the trust is funded, but the tax is not actually owed until the skip-generation transfer occurs decades later. A poor allocation decision in 2026 can produce a 40% tax bill in 2070 that was structurally avoidable.

This article is the working note for wealth-tech platforms serving dynasty-trust planning, on what the data model needs to track, why the inclusion ratio is the keystone field, and what the planning engine has to support.

The mechanics

A skip person is a beneficiary two or more generations below the transferor. Grandchildren are skip persons. Great-grandchildren are skip persons. Trusts can be skip persons (a trust whose beneficiaries are all skip persons is itself a skip person; the GST tax fires when any taxable termination, taxable distribution, or direct skip occurs).

Three taxable events trigger GST:

• Direct skip: a transfer directly to a skip person. Grandparent gifts $100K to grandchild — direct skip.
• Taxable distribution: a distribution from a trust to a skip-person beneficiary. Trust pays $50K to grandchild — taxable distribution.
• Taxable termination: a non-skip beneficiary's interest in a trust terminates (typically at their death) and only skip-person beneficiaries remain. Parent (the income beneficiary) dies and the trust now serves the grandchildren — taxable termination.

The tax is 40% on the value of the transferred property. The exemption applies to reduce the taxable amount.

The inclusion ratio — the keystone field

The inclusion ratio is the central concept. For a trust, the inclusion ratio is the fraction of the trust's value that's subject to GST tax at the skip event. A trust with inclusion ratio 0 is fully GST-exempt; a trust with inclusion ratio 1 is fully GST-taxable; partial allocations produce ratios between 0 and 1.

The inclusion ratio is computed at the time of the transfer (or the time of the late allocation if applicable) using:

inclusion_ratio = 1 − (GST_exemption_allocated / value_of_transferred_property)

Once set, the inclusion ratio is fixed for that transfer. Subsequent appreciation in the trust is included in or excluded from GST in the same proportion as the original property.

Example: a donor transfers $5M to a dynasty trust and allocates $5M of GST exemption. Inclusion ratio = 1 − ($5M / $5M) = 0. The trust is fully GST-exempt. If the trust appreciates to $50M over decades, the entire $50M is exempt at the skip event.

Same donor instead allocates $3M of GST exemption to a $5M transfer. Inclusion ratio = 1 − ($3M / $5M) = 0.4. Forty percent of the trust's distributions / terminations are subject to 40% GST tax. If the trust appreciates to $50M, 40% × $50M = $20M is exposed to GST, generating $8M of tax.

The leverage is enormous. The same $5M of GST exemption used to fully exempt a smaller transfer (and let it appreciate exempt) is more efficient than partially exempting a larger transfer.

interface GSTAllocation {
  transferor: Person;
  transfer_id: string;
  transfer_date: ISODate;
  transfer_type: 'direct_skip' | 'trust_funding' | 'late_allocation';
  
  // Transfer properties
  property_value_at_transfer: number;
  property_basis: number;
  receiving_trust_id?: string;
  
  // GST exemption allocation
  exemption_allocated: number;
  inclusion_ratio: number; // Computed: 1 - (allocated / value)
  
  // Lifetime GST exemption tracking
  transferor_lifetime_exemption_total: number;
  transferor_lifetime_exemption_used_before_this: number;
  transferor_lifetime_exemption_used_after_this: number;
  
  // For automatic allocation (default rule)
  automatic_allocation_applied: boolean;
  automatic_allocation_opt_out: boolean;
  
  // Late allocation (if applicable)
  late_allocation: {
    original_transfer_date: ISODate;
    late_allocation_date: ISODate;
    value_at_late_allocation_date: number;
  } | null;
}

The transferor_lifetime_exemption_* fields track the donor's running total. The exemption is per-donor over their lifetime; once used, it's used.

The automatic-allocation default

Code §2632 provides that GST exemption is automatically allocated to "indirect skips" (transfers to a GST trust where the donor doesn't take affirmative action otherwise) unless the donor opts out. This default rule is a trap as often as it's a help.

A donor who funds a GST trust with $5M and assumes their estate planning attorney handled the allocation may discover, years later, that the automatic allocation applied to other transfers as well — depleting the lifetime exemption pool unintentionally.

A wealth-tech platform that tracks the donor's lifetime GST exemption status should surface the running balance, the transfers that have triggered automatic allocation, and the transfers where opt-out elections were filed. Without this surfacing, the donor flies blind on their exemption pool.

The dynasty trust as the planning vehicle

Dynasty trusts (also known as long-term trusts, perpetual trusts, or generation-skipping trusts) are the vehicle of choice for GST planning. The trust is funded with GST-exempt property, and it can persist for multiple generations, with the GST exemption protecting all distributions and terminations along the way.

State law governs how long the trust can persist. Twenty-six states have repealed or substantially extended the rule against perpetuities, allowing trusts to persist indefinitely or for very long terms (often 360+ years). Delaware, Nevada, Alaska, South Dakota, and others are the popular dynasty-trust jurisdictions.

The platform must capture the trust's situs and the applicable state's perpetuities rule. A trust with a 21-year + life-in-being rule may be structurally limited; a trust in a perpetuities-repealed state can in principle support indefinite dynasty planning.

The state choice has additional consequences: state income tax on undistributed trust income, state-level grantor-trust treatment, asset-protection law (some states have stronger creditor protection for trust assets). The platform's modeling should include the state-specific overlay.

Late allocation and the value-at-allocation gotcha

A donor who didn't allocate exemption at the original transfer can sometimes allocate retroactively (a "late allocation"). The catch: the inclusion ratio is computed using the value of the trust at the late-allocation date, not the original transfer date.

For a trust that has appreciated substantially, late allocation requires more exemption than original allocation would have. A donor who funded a trust with $5M in 2020 (and didn't allocate exemption) might find in 2026 that the trust is now worth $12M — to fully exempt the trust now requires $12M of exemption allocation, not $5M.

The platform should surface the late-allocation analysis: how much exemption is needed today to fully exempt the trust, vs. how much was needed at original transfer. The delta represents the cost of the original failure to allocate.

The married-couple coordination

Married couples have separate GST exemptions ($13.99M each in 2025). Coordination between spouses is consequential: which spouse's exemption is used for which trust, and how does the surviving spouse's portability of the deceased spouse's unused exemption (DSUE) interact with GST?

Generally, GST exemption is not portable between spouses. Unlike the gift / estate tax exemption (which is portable via the DSUE election), GST exemption that's unused at the first spouse's death is lost. This is a frequently-missed planning surface — the dying spouse's unused GST exemption needs to be allocated before death, often through last-minute trust funding or through specific planning structures (formula clauses in the will).

The platform should track each spouse's GST exemption balance separately and surface optimization recommendations: which spouse should fund which trust, when to use exemption vs. preserve for later.

What the test corpus needs

A test corpus for GST modeling needs:

• Donors with significant GST exemption headroom (planning opportunity)
• Donors with substantially-used GST exemption (allocation-budgeting case)
• Trusts at funding stage (allocation decision)
• Trusts with partial allocation (inclusion ratio between 0 and 1)
• Trusts approaching skip events (taxable termination horizon)
• Late-allocation candidates (after-the-fact exemption use)
• Married-couple coordination scenarios (per-spouse exemption tracking)
• Multi-state trust portfolios (situs choice)
• Trusts that have exhausted their exempt portion through distributions (cap-tracking)

Each scenario produces a structurally different planning recommendation. A platform that exercises only "fund the trust, allocate full exemption" misses the partial-allocation, late-allocation, and coordination scenarios where the planning expertise actually lives.

The advice surface

A platform that models GST correctly can surface advice along multiple dimensions:

• "You have $X of GST exemption headroom. Funding additional dynasty trusts with appreciated assets now and allocating exemption produces lifetime tax savings of $Y at expected appreciation rates."
• "Your dynasty trust funded in 2020 has appreciated; the inclusion ratio is now 0.6. Late allocation today would require $Z of additional exemption. Without late allocation, the projected GST at the skip event is $W."
• "Your spouse has $X of unused GST exemption that is not portable at death. Pre-death funding strategies to consider..."
• "Your dynasty trust's situs in [state] limits the perpetuities to [Y] years. Decanting to a perpetuities-repealed jurisdiction could extend the trust's effective duration."

These advice surfaces require the data model. Without the model, the platform's GST-related output is generic and the donor is back to "ask your estate planning attorney" — which is true but doesn't help the donor make the time-sensitive allocation decisions.

The synthetic-corpus value

Realistic GST and dynasty-trust data is rare in any production dataset because the structures are concentrated in HNW populations and rarely visible to wealth-tech feeds. A purpose-built synthetic corpus with documented exemption-use history, inclusion-ratio state, and trust-situs information is required for the planning engine to be tested rigorously.

The corpus serves as the template for production data capture. If the corpus tracks per-transferor lifetime exemption, the platform's data model should too. If the corpus distinguishes automatic-allocation vs. opt-out elections, the platform's UI should ask. Without these fields in production, the platform is structurally unable to advise on the GST allocation decisions that determine multi-generational outcomes.

GST planning is the apex of estate-planning sophistication. Wealth-tech platforms that handle it correctly differentiate themselves at the high end of the HNW market where the structures are concentrated. Without the model, the platform's GST-related output is generic — and for a $20M dynasty trust, generic output produces $5-10M of structurally avoidable tax over the planning horizon. The data model is the difference.