Research · Altcoins · Strand B

ETH behaviour during Bitcoin bull phases

When Bitcoin enters a sustained advance, the wider market usually advances too. This page describes that association carefully: what a bull phase is as a defined window, what co-movement within it can and cannot show, and why a common market factor is the more parsimonious explanation than one asset leading the other.

Last reviewed 2026-09-21Source: CoinGecko historical price data; Bitcoin cycle definitions from this site's cycle recordAssociation and method only. No return figure is asserted for any phase.

Defining a bull phase

A bull phase has to be defined before anything can be said about what happened during one. The definition used here is the one this site applies to Bitcoin's own record: a sustained advance from a cycle low to the subsequent cycle high, with the dates fixed after the fact by the price series rather than chosen in advance. That definition is retrospective by construction, and it is worth being explicit about the consequence. A phase boundary drawn at a peak is only knowable once the peak has passed, so any statement of the form "ETH did X during Bitcoin's bull phase" is a statement about a window that could not have been identified while it was open.

The site's own cycle record is the reference for those boundaries. The Cycles page sets out the expansion and contraction phases with their dates, and the Cycle Comparison page puts the completed cycles side by side. Using the same boundaries here keeps this page consistent with the rest of the site rather than introducing a second, incompatible definition.

A second definitional choice is what "ETH behaviour" means. It can mean the absolute return of ether over the window, the return relative to bitcoin, or the path the price took including its drawdowns. These are different questions with different answers, and a page that switches between them without saying so will appear to contradict itself. This page uses relative performance against bitcoin as its primary measure, because that is the comparison the strand is about.

Association within a common market factor

What co-movement between ETH and BTC during a Bitcoin bull phase can and cannot establish.
Observation	What it supports	What it does not establish
Both assets advanced over the same window	Association: the two series moved in the same direction over a shared period	That Bitcoin's advance caused Ethereum's; a shared driver produces the same pattern
ETH outperformed BTC over the window	Relative performance: ether gained more than bitcoin against the same reference	That the outperformance was caused by Bitcoin's advance rather than by ETH-specific factors
ETH's advance appeared to follow BTC's	Timing: one series moved before the other at the resolution examined	Causation; lead-lag at daily resolution is unstable and often reverses at other frequencies
ETH failed to advance while BTC did	Decoupling: an asset-specific factor was strong enough to offset the common one	That the common factor is absent in other periods, or that the decoupling will persist

Last reviewed 2026-09-21Source: Method description; phase boundaries from this site's cycle recordNo phase return is stated; the table separates what each observation supports.

The parsimonious explanation for broad co-movement during a Bitcoin advance is a common market factor rather than a causal chain running from one asset to the other. The two networks share an investor base, a settlement currency, a set of trading venues and a liquidity cycle. When risk appetite for the asset class rises, capital tends to enter the largest and most liquid asset first and then spread outward. That mechanism produces co-movement without requiring Bitcoin's price to cause Ethereum's.

This is not a claim that no causal relationship exists. It is a claim about what the evidence can distinguish. Co-movement is consistent with a common factor, with one asset leading the other, and with a third variable driving both. Distinguishing between those requires more than a correlation over a shared window, and the honest position is that the co-movement alone does not decide it.

The lead-lag observation deserves particular caution. At daily resolution, one asset frequently appears to move first, and that appearance is unstable: it changes with the sampling frequency, the window and the period examined. An apparent lead that reverses when the data is resampled is not evidence of a causal ordering. For the underlying cycle structure, see Cycle Comparison and Return Profiles.

The limits of the comparison

The number of completed Bitcoin bull phases is small. Each one is a single observation, and a handful of observations cannot support a general rule about how another asset behaves during them. Any pattern described across three or four phases is a description of those phases, not a law. This is the central limitation of the whole strand, and it is stated on every page rather than only here.

The phases also differ from each other in ways that matter. The market structure around them changed: the venues, the instruments, the leverage available and the participant base were not the same in each. A pattern that held in one phase may reflect the market structure of that period rather than a durable relationship. For the mechanics of how a drawdown is measured, see Drawdown Explained.

Dataset, period, method and limitations

Dataset: Daily ETH and BTC closing prices from CoinGecko. Phase boundaries are taken from this site's own cycle record so the windows match the rest of the site.
Period: From Ethereum's public trading history to the last-reviewed date stated above, covering the completed Bitcoin cycles within that span.
Method: Each Bitcoin bull phase is treated as a fixed window defined retrospectively by the price series. ETH performance is measured relative to BTC over the same window. Observations are classified as co-movement, relative performance, timing or decoupling, and each is reported with what it does and does not support.
Limitations: The sample of completed phases is very small, so no pattern can be generalised from it. Phase boundaries are retrospective and could not have been identified in real time. Co-movement is consistent with a common factor, with reverse causation and with a third driver, and this method cannot distinguish between them.

What this page does not claim

This page does not state a return for any phase, and it does not claim that Bitcoin's advance caused Ethereum's. It does not present an apparent lead-lag as evidence of causation, and it does not generalise from a handful of completed cycles to a rule about future behaviour.

Sources and references

Every source below is named and linked. Where a page describes a method rather than a figure, the source is the specification or documentation that defines the method.

Bitcoin historical data. CoinGecko, www.coingecko.com/en/coins/bitcoin/historical_data: the series from which phase boundaries are drawn.
Ethereum historical data. CoinGecko, www.coingecko.com/en/coins/ethereum/historical_data: the ETH leg of the relative-performance measure.
Community data. Coin Metrics, coinmetrics.io/community-network-data/: an independent series used to cross-check the price inputs.
Bitcoin market cycles. Bitcoin Data Guide, bitcoindataguide.com/cycles: the phase definitions this page reuses rather than redefining.