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On the Expressiveness of Approximate Inference in Bayesian Neural Networks

T0 review · 1 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read For one-hidden-layer ReLU Bayesian neural networks, mean-field Gaussian variational inference and Monte Carlo dropout provably cannot represent the increased uncertainty between well-separated data clusters that exact Bayesian inference…

desk verdict First proof that mean-field Gaussian and MC dropout posteriors provably lack in-between uncertainty in shallow ReLU BNNs; a solid, citable negative result with one minor appendix overstatement. read the letter →

arxiv 1909.00719 v4 pith:O2HCSKFH submitted 2019-09-02 stat.ML cs.LG

classification stat.MLcs.LG MSC 62F1568T07
keywords Bayesianneuralnetworksvariationalinferencemean-fieldapproximationMonteCarlodropoutpredictiveuncertaintyin-betweenReLUactivelearning
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper sets out to show that two of the most common approximate-inference methods for Bayesian neural networks — mean-field Gaussian variational inference and Monte Carlo dropout — can, even in the simplest one-hidden-layer ReLU network, fail to reproduce a basic qualitative feature of the exact Bayesian predictive distribution: the rise in uncertainty that should occur in the gap between two well-separated clusters of data. It proves this as a theorem about the weight-space structure of the approximating families, and it shows empirically that exact inference, represented by the infinite-width Gaussian process and Hamiltonian Monte Carlo, does exhibit that in-between uncertainty on the same problem. The paper also proves that the limitation is not fundamental to depth: with at least two hidden layers, both families contain distributions whose predictive mean and variance can approximate any continuous target functions. Yet when the usual variational objectives are optimized in deep networks, the same pathological confidence persists, and in an active-learning case study it leads MFVI and MCDO to choose points worse than random selection while the Gaussian-process reference gains a threefold improvement. The conclusion is that failures in BNN uncertainty estimates can be caused by the approximation family and the optimization objective, not by the model or prior.

What carries the argument

The load-bearing object is the decomposition of predictive variance by the law of total variance, conditioned on the first-layer parameters. Under the factorised families, the conditional variance of the output is $V[f(x) | \theta_{in}] = \sum_i V[w_i] \psi(a_i(x))^2 + V[b]$, where $\psi(a) = \max(0, a)$ is the ReLU; because $\psi^2$ is convex, this term is convex in $x$, which alone proves Theorem 2 and explains the non-convexity that could rescue Theorem 1. The remaining term for mean-field Gaussian, $V[\mathbb{E}[f | \theta_{in}]]$, is a positive linear combination of variances of rectified activations, and the appendix proves these satisfy reflection and monotonicity properties (sets $T_1$ and $T_2$) that force the line-segment bound of Theorem 1. Depth enters through a construction that splits the two units of the second hidden layer into a mean-control channel and a variance-control channel, using the deterministic universal approximation theorem for ReLU networks to argue that both the approximate mean and variance can match any continuous targets.

What would settle it

A single counterexample would settle the main claim: find a single-hidden-layer ReLU network whose weights follow a fully factorised Gaussian or an MC dropout distribution (inputs not dropped), pick two points $p$ and $q$ satisfying the conditions of Theorem 1, and observe strictly larger predictive variance at an interior point $r$ than the sum of the endpoint variances; the theorem says this is impossible. For the empirical half, one can repeat the two-cluster regression with three clusters arranged so the middle cluster is unobserved: the paper predicts Hamiltonian Monte Carlo and the infinite-width GP will show elevated uncertainty at the unobserved middle cluster while MFVI and MCDO will not, across all hyperparameter settings.

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Extended reading notes

Core claim

For a single-hidden-layer fully connected ReLU network, any approximate posterior drawn from the fully factorised Gaussian family or from the MC dropout family (with inputs not dropped out) has a predictive variance that cannot be substantially larger between well-separated regions of low uncertainty. Theorem 1 proves that on line segments passing through the origin, or crossing an axis plane, the variance at an interior point is bounded by the sum of the variances at two flanking endpoints, $V[f(r)] \le V[f(p)] + V[f(q)]$; Theorem 2 proves the stronger statement that for MC dropout the variance function is convex in the input, so on any segment it is bounded by the maximum at the endpoints. The paper provides strong empirical evidence that the exact posterior does not share this pathology: the infinite-width Gaussian process and Hamiltonian Monte Carlo posteriors show visibly elevated uncertainty between two data clusters. For networks with at least two hidden layers, Theorem 3 states that the predictive mean and variance of both approximate families are universal approximators, yet ELBO-optimised deep MFVI and MCDO still exhibit the same overconfidence between clusters, and the active-learning experiment on the Naval dataset shows MFVI and MCDO degrading performance relative to random acquisition.

Load-bearing premise

The proofs assume the approximate posterior factorises across hidden neurons, so the output weights attached to different neurons are independent and carry no negative correlations; if correlations between those weights were allowed, the variance could become non-convex and the bounds in Theorems 1 and 2 would no longer follow.

Editorial extensions

If this is right

  • For single-hidden-layer ReLU BNNs, no choice of variational parameters, prior, or optimiser can make a fully factorised Gaussian or MC dropout posterior represent in-between uncertainty on the line segments covered by Theorems 1 and 2; the failure is a property of the approximating family itself.
  • Any inference method that uses the fully factorised Gaussian family — including diagonal Laplace and other variational schemes — inherits the same function-space limitation, because Theorem 1 depends only on the family.
  • The exact posterior of the BNN model does show increased uncertainty between well-separated data, so the lack of in-between uncertainty in the approximations is not a modelling defect; it is introduced by approximate inference.
  • In deep networks, theoretical expressiveness alone does not guarantee good approximate inference: the universality result is real, but ELBO-optimised MFVI and MCDO empirically remain overconfident between data clusters, even when initialised from a function-space fit that already has the correct uncertainty shape.
  • In the active-learning case study, the exact-inference GP reduces test RMSE by roughly a factor of three relative to random selection, whereas 1HL MFVI and MCDO increase RMSE and deeper variants mostly fail to beat random selection, showing a concrete practical cost of the pathology.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The core mechanism — independence of outgoing weights cancels the covariance terms that make variance non-convex — suggests the same obstruction applies to any factorised family whose activations have a convex squared non-linearity, and to deeper variants that use such factorisation in the final layer; a testable prediction is that a full-covariance Gaussian posterior with negative correlations be
  • A natural extension would be to measure the 'distance-to-origin bias' of MFVI predictive variance in high-dimensional input spaces: the Naval result implies that even where the theorem's precise line conditions are rare, the family still ranks uncertainty more by distance from the origin than by local data density, and this ranking could be quantified directly.
  • The deep-network result separates expressiveness from optimisation: Theorem 3 says good distributions exist in the family, while the ELBO experiments say they are not found. This suggests a testable diagnostic for new variational objectives: initialise from the squared-loss solution and check whether ELBO maximisation preserves or destroys in-between uncertainty.
  • If the pathology is caused by the factorised family and the ELBO landscape, then non-factorised approximations (for example, inducing correlations between output weights) or alternative objectives that directly match function-space moments should eliminate the overconfidence pattern in the same toy dataset; this is a direct, checkable implication.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

1 major / 4 minor

Summary. The paper studies the function-space expressiveness of mean-field Gaussian (QFFG) and Monte Carlo dropout (QMCDO) approximating families for Bayesian neural networks. The central theoretical result is that for single-hidden-layer ReLU BNNs, QFFG and QMCDO cannot represent 'in-between uncertainty' in the predictive variance: Theorem 1 bounds the variance at a point between two well-separated low-uncertainty regions on certain line segments, and Theorem 2 shows the MCDO variance is convex. The authors separate Criterion 1 (does the family contain good approximations to the posterior?) from Criterion 2 (does optimization find them?). Empirical comparisons against HMC and the infinite-width GP on a two-cluster regression task show that the exact posterior does exhibit in-between uncertainty, while MFVI and MCDO do not. For two-hidden-layer networks, Theorem 3 gives universality of predictive mean and variance for both families, but ELBO optimization in deeper networks still shows overconfidence between clusters (Section 4.2). A final active-learning case study on the Naval dataset shows GP-based selection succeeds while MFVI/MCDO-based selection degrades RMSE relative to random selection.

Significance. If the results hold, this is an important contribution to the BNN approximation literature: it gives a crisp, weight-space-to-function-space explanation of a known qualitative failure of two widely used variational families, and it carefully separates model quality from approximation quality using HMC and the infinite-width GP as references. The theorems are proven in detail with standard tools (law of total variance, convexity of rectified quadratics, Chebyshev and union-bound arguments), and the paper ships code for reproduction. The universality result for two-hidden-layer networks is a useful counterpoint showing that depth can in principle restore expressiveness, and the explicit constructive proofs for both QFFG and QMCDO are a strength. The empirical protocol—direct squared-loss minimization in function space, ELBO-based fitting, overconfidence-ratio boxplots, and active learning—triangulates the theoretical claims well. The main caveat is that the exact-posterior claim is empirical rather than proven, and the shallow-case theorems cover special line configurations; these limitations are acknowledged in the paper.

major comments (1)
  1. [Appendix B, Theorem 4; Appendix E.1, Theorem 8] Theorem 4 is stated for arbitrary conditional densities q_i(w_i|U,v) and q(b|U,v), but the proof's equation (5) treats E[w_{k,i}|U,v] and E[b_k|U,v] as constants independent of U,v. If the conditional densities are truly allowed to depend on U,v, then V[E[f|U,v]] = V[sum_i μ_{w_{k,i}}(U,v)ψ(a_i) + μ_{b_k}(U,v)] contains additional covariance terms that are not accounted for. The same gap appears in Theorem 8, where V[E[f(0)|U]]=0 is asserted although q_i(w_i|U) and q(b|U) are allowed to depend on U. Both gaps disappear for the actual QFFG and MCDO families, whose conditional means are constant, so Theorems 1 and 2 survive; nevertheless, the statements of Theorems 4 and 8 should be restricted (e.g., to q_i(w_i|U,v)=q_i(w_i) and q(b|U,v)=q(b)) or the proofs extended to handle the general case.
minor comments (4)
  1. [Section 4, paragraph before Theorem 3] The sentence 'this provides theoretical evidence that approximate inference in deep BNNs satisfies Criterion 1' is stronger than the theorem warrants: Criterion 1 is about containing good approximations to the actual posterior, whereas Theorem 3 only constructs elements of the family with prescribed first two output moments on a compact set. The caveats in the following paragraphs mitigate this, but the wording should be qualified.
  2. [Appendix F.1] The prior-scaling procedure is described by the list of weight standard deviations and the statement that prior standard deviations in function space are 'between 10 and 15'; a more precise statement of the target value or the validation criterion would improve reproducibility.
  3. [Section 5 and Appendix H.1] The active-learning experiment fixes the observation-noise standard deviation to 0.01 for all models and all depths; a brief sensitivity check for this choice would strengthen the comparison, since the GP reference and the BNNs share the same fixed noise.
  4. [Figure 5 caption; Appendix D] The Figure 5 caption contains a duplicated article ('and the the box extends'), and several displayed sup-norm inequalities in Appendix D contain duplicated vertical-bar characters (e.g., '|||...|||∞'); these formatting issues should be cleaned up.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the limitation theorems are proved from the defining structure of QFFG/QMCDO and checked against independent HMC/GP references.

full rationale

The central claims are derived from the definitions of the approximating families, not from the target conclusion. Theorem 1 and Theorem 2 use the conditional independence of outgoing weights (equation (2) and Appendix C.1 equation (6)) to expand the conditional variance, then apply non-trivial lemmas about rectified-Gaussian variances and convexity; the bound is therefore a consequence of the family's factorization, not an assumption of the result being proved. The claim that exact inference lacks the pathology is supported empirically by HMC and by the infinite-width GP, so no fitted parameter is relabelled as a prediction. The deep-network universality theorem is an existence construction using the external universal-approximation theorem (Leshno et al.) with explicit parameter choices, and the paper separately checks whether ELBO optimization actually finds such distributions (Criterion 2). Self-citations are present but not load-bearing: the GP-limit references include external independent results (Neal; Lee et al.), and the in-house citations are used only for initialization practice or as examples of methods using QFFG. One non-circular caveat is worth flagging: Appendix B's Theorem 4 is stated for arbitrary conditional densities q_i(w_i|U,v), but its proof implicitly treats E[w_i|U,v] as constant in U,v when deriving equation (5); for the fully-factorised Gaussian family used in the main text this condition holds, so Theorems 1 and 2 are unaffected. This is a correctness/generalization gap in a supporting statement, not a circular reduction of the paper's main argument.

Assumptions & free parameters 0 free parameters · 5 assumptions · 0 invented entities

The theoretical claims rest on standard probability results and the universal approximation theorem. The empirical comparison to 'exact inference' uses the GP-limit and HMC as external references, which are well-supported but not proven in this paper.

assumptions (5)
  • standard math Universal approximation by deterministic ReLU networks (Leshno et al., 1993)
    Used in the proof of Theorem 3 to approximate the target mean g and variance h by the activations of a 2-hidden-layer network; see Appendix D, proof of Theorem 6.
  • standard math Law of total variance and conditional independence of output weights under factorized variational families
    Used in proofs of Theorems 1, 2, 4, and 5; see Section 3.1, equation (2), and Appendix C.1, equation (6).
  • standard math Variance formula for a rectified Gaussian unit (Frey and Hinton, 1999)
    Used in Lemma 2 to characterize the variance of a single neuron's output as a function of input; see Appendix C.2.
  • domain assumption Infinite-width BNNs converge to Gaussian processes and moderate-width BNNs resemble their GP limit under the chosen prior scaling
    Used as the reference for exact inference in the empirical comparisons (Figures 3, 5, 6); see Section 2.2.
  • domain assumption Hamiltonian Monte Carlo samples are representative of the exact posterior
    Used as a gold-standard reference for exact inference in the 1HL and 2HL cases; see Section 3.2 and Appendix F.

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Cite this review

Pith. "Pith review of On the Expressiveness of Approximate Inference in Bayesian Neural Networks." pith.science (2026). https://pith.science/paper/O2HCSKFH

@misc{pith2026190900719,
  author       = {Pith},
  title        = {Pith review of: On the Expressiveness of Approximate Inference in Bayesian Neural Networks},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/O2HCSKFH}},
  note         = {Machine review of arXiv:1909.00719}
}
read the original abstract

While Bayesian neural networks (BNNs) hold the promise of being flexible, well-calibrated statistical models, inference often requires approximations whose consequences are poorly understood. We study the quality of common variational methods in approximating the Bayesian predictive distribution. For single-hidden layer ReLU BNNs, we prove a fundamental limitation in function-space of two of the most commonly used distributions defined in weight-space: mean-field Gaussian and Monte Carlo dropout. We find there are simple cases where neither method can have substantially increased uncertainty in between well-separated regions of low uncertainty. We provide strong empirical evidence that exact inference does not have this pathology, hence it is due to the approximation and not the model. In contrast, for deep networks, we prove a universality result showing that there exist approximate posteriors in the above classes which provide flexible uncertainty estimates. However, we find empirically that pathologies of a similar form as in the single-hidden layer case can persist when performing variational inference in deeper networks. Our results motivate careful consideration of the implications of approximate inference methods in BNNs.

Figures

Figures reproduced from arXiv: 1909.00719 by the authors.

Figure 1
Figure 1. Illustration of the bounded regions in theorem 1, showing the input domain of a 1HL [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Results of directly minimising the squared error in function space between V[f(x)] (for a single-hidden layer NN) and a target variance function. Left: FFG distribution, Right: MCDO distribution. The bound for FFG distributions (red) applies on [−1, 1] with p = −1, q = 1. The MCDO variance function is convex, and almost constant. The FFG and MCDO variance functions underestimate the target near the origin and overes… view at source ↗
Figure 3
Figure 3. Regression on a 2D synthetic dataset (red crosses). The colour plots show the standard [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (12 more)
Figure 4
Figure 4. Figure 4: Results of minimising the squared error in function space between [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]
Figure 5
Figure 5. Figure 5: Box and whisker plots of the overconfidence ratios of HMC, MFVI and MCDO relative [PITH_FULL_IMAGE:figures/full_fig_p007_5.png]
Figure 6
Figure 6. Figure 6: Points chosen during active learning in the 1HL case. Colours denote distance from the [PITH_FULL_IMAGE:figures/full_fig_p008_6.png]
Figure 7
Figure 7. Figure 7: Mean and 2 standard deviation bars of the predictive distribution on lines joining random [PITH_FULL_IMAGE:figures/full_fig_p013_7.png]
Figure 8
Figure 8. Figure 8: Same experimental set-up as in figure 7 for the 3HL case. [PITH_FULL_IMAGE:figures/full_fig_p014_8.png]
Figure 9
Figure 9. Figure 9: Schematic illustration of the bound in theorem 2, showing the input domain of a single [PITH_FULL_IMAGE:figures/full_fig_p028_9.png]
Figure 10
Figure 10. Figure 10: figure 10. With this prior both methods tend to be less over-confident between data clusters, but more [PITH_FULL_IMAGE:figures/full_fig_p030_10.png]
Figure 11
Figure 11. Figure 11: Plots of the overconfidence ratio against [PITH_FULL_IMAGE:figures/full_fig_p030_11.png]
Figure 12
Figure 12. Figure 12: Mean and error bars (± 2 standard deviations) for the GP and the BNN with each inference scheme, trained on the data shown by the red crosses. The inference algorithms were initialised by first minimising the squared error to the reference GP mean and variance, and th…
Figure 13
Figure 13. Figure 13: Points chosen during active learning in the 3HL case. Colours denote distance from the [PITH_FULL_IMAGE:figures/full_fig_p033_13.png]
Figure 14
Figure 14. Figure 14: Colours denote predictive uncertainties in the 1HL case, at the beginning of active learning. As the noise standard deviation was fixed to 0.01 for all models, changes in the predictive standard deviation reflect model uncertainty. Grey crosses (✕) denote the five poi…
Figure 15
Figure 15. Figure 15: Colours denote predictive uncertainties in the 1HL case, after 50 iterations of active learning. As the noise standard deviation was fixed to 0.01 for all models, changes in the predictive standard deviation reflect model uncertainty. Grey crosses (✕) denote the five …

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    Finally, applying lemma 5, we have V[ψ(f (k) θ (x))]<ϵ for 1≤k≤K

    Then V[f (k) θ (x)]≤I 2 ϵ 2I 2 +σ′2 <ϵ for 1≤k≤K. Finally, applying lemma 5, we have V[ψ(f (k) θ (x))]<ϵ for 1≤k≤K. The following lemma states that by setting the standard deviation of the weights to be sufficiently small, we can with high probability make the sampled BNN outpu...

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Reviewed August 14, 2026 · model on record in the stance chip above.