Personal Finance Day 6

The Credit System (US)

A credit score isn't a morality score—it's an algorithm you can reverse-engineer. Understand how it's computed and you can raise it at almost no cost.

个人理财 · 信用体系（美国）　|　2026-07-21

Most people think a credit score measures how much money you have. It actually measures how reliably you repay—and that number directly sets the price you pay to borrow: mortgage rates, auto loans, credit cards, and in many US states even rental approvals and car-insurance premiums. Here's the uncomfortable part: on the same 30-year mortgage, a gap of a few dozen points can mean a five-figure difference in interest. The good news is that it's an algorithm, not black magic. This issue nails four things: how the five FICO factors are computed, how to build and optimize credit cheaply, whether you're playing the rewards game or being played by it, and identity-theft protection (with US–China differences flagged).

Four Foundations

POINT 1

How FICO Is Built: Five Factors, Two Own 65%

Your score is a weighted sum of five factors—two of which make up 65%. Focus on those two.

Mechanism. FICO (Fair Isaac Corporation, the dominant US consumer credit score) ranges 300–850. It compresses your credit report into a weighted sum of five factors: payment history and credit utilization together account for 65%—so "pay on time" and "don't max out" nearly decide the whole thing.

Factor	Weight	How to optimize
Payment history	35%	Never miss, not once
Credit utilization	30%	Keep under 30%, ideally <10%
Length of history	15%	Don't close old cards
New credit / hard inquiries	10%	Apply rarely, don't cluster
Credit mix	10%	Let it happen; don't force it

Utilization = balance used ÷ total limit. Rough bands: 670+ is "good," 740+ "very good," 800+ "exceptional." Exact numbers vary by FICO version; the direction is stable.

Common myth: "Checking my own score lowers it." — It doesn't. Checking your own score is a soft pull and never affects it; what dings you is a hard pull, triggered when a lender pulls your report for a new application. Check your own score as often as you like.

This week: Pull your FICO / VantageScore for free (most bank and card apps now offer it) and note the one factor flagged red. Reflection: is your weakest factor a habit problem or just a time problem?

POINT 2

Build and Optimize: Three Free Levers

Building credit takes time and consistency; optimizing it runs on three free levers: on-time, utilization, age.

Mechanism. Credit is a track record proving you can borrow responsibly—starting from zero takes time. But raising it costs almost nothing; you just manage three things.

Actionable.

Start from zero: use a secured card (your deposit is your limit), or have a credit-healthy family member add you as an authorized user to piggyback on their age and history.
100% on time: even if you only pay the minimum, never miss—automate it. Lateness reports at 30 days past due, but a single 30-day late can knock off 50–100 points and linger for years.
Lower utilization: spread balances across cards, request limit increases, and pay down before the statement closes—it's the statement snapshot that counts, not month-end.
Don't close your oldest card: closing cuts your total limit (raising utilization) and, years later, lowers your average account age.
Open new credit sparingly: each hard inquiry is roughly −5 to −10 points for about 12 months; but multiple inquiries for the same mortgage/auto loan within a 14–45 day window count as one.

Common myth: "Carrying a small balance builds credit." — Expensively wrong. Building credit only needs you to pay in full, on time; carrying a balance just donates interest (card APR—annual percentage rate—often runs 20–24%). Utilization is a statement-date snapshot; it has nothing to do with whether you leave a balance.

This week: Set autopay-in-full on every card; confirm your oldest card is open and kept active with an occasional small charge. Reflection: have you ever closed an old card to "save on fees" and quietly lost account age?

POINT 3

Credit-Card Rewards: Playing vs. Being Played

Only people who pay in full are playing the rewards game; carry a balance and the rewards are playing you.

Mechanism. Cash back / points (1–5%) are bait the issuer funds from merchant fees and from the interest that balance-carriers pay. The bank's real profit is interest and annual fees. The math is brutal: earn 2% back while carrying a 22% APR balance and you're down ~20% a month—the rewards don't even cover the rounding error.

Actionable.

Iron rule: touch a rewards card only if you can pay in full every month; if you can't, use the plainest card and focus on paying down debt.
Cash back vs. travel points: cash back is simple and certain; travel points have a higher ceiling but must be redeemed skillfully and get devalued (issuers reprice redemptions at will).
Annual-fee cards: worth it only if you genuinely use up the perks (on spending you'd do anyway); otherwise the fee exceeds the rewards.
Never spend for points: spending $1,000 to "save $100" is the oldest marketing trap there is.

Common myth: "I put more on the card to rack up miles." — Unless that money was going to be spent anyway, you paid 100% of the principal to chase a 1–5% rebate. Rewards should never drive spending—they should only recapture value from spending you'd have done regardless.

This week: Pick one card and compute: annual fee − (cash back + perks you actually used over the last 12 months) = positive or negative? If negative, downgrade or close it. Reflection: how much of your spending has been quietly inflated by "earning points"?

POINT 4

Identity-Theft Protection (with US–China Differences)

The strongest protection is free: turn on a credit freeze at all three bureaus.

Mechanism. Identity theft = someone using your identity to open credit and rack up debt in your name. The US has three bureaus: Equifax, Experian, TransUnion. A credit freeze has been free at all three since 2018—once frozen, no one (including you) can open a new account with your information until you temporarily lift it. That's a lock placed before the fact, far stronger than an after-the-fact alert.

Actionable.

Freeze at each of the three bureaus (about 5 minutes each, one-time); temporarily lift it when you genuinely apply for credit, then re-freeze.
Use the official AnnualCreditReport.com to pull all three reports for free on a schedule (now available weekly) and check for accounts you don't recognize.
Lighter alternative: set a fraud alert, requiring lenders to verify your identity before opening credit in your name.
US–China difference: China has no FICO. Personal credit is centrally managed mainly by the PBOC Credit Reference Center (People's Bank of China); private scores like Sesame Credit are proprietary and are not the official credit file. Inquiry, dispute, and "freeze" processes all differ from the US. Credit built in the US does not carry over to China, and vice versa—cross-border movers build from scratch on both sides.

Common myth: "A credit-monitoring service protects me from identity theft." — Monitoring is only an after-the-fact alert; a freeze is a before-the-fact lock—and it's free. Many paid monitoring products are selling you something you can do for free.

This week: Turn on a credit freeze at all three bureaus today (5 minutes each). Reflection: if someone opened a card in your name tomorrow, how soon would you find out?

Note for High-Earning Tech Workers

One per issue, focused on the specific situation of this group: equity comp / heavy taxes / high liquidity.

"Fat wallet, thin credit file" is the classic trap for high-earning new immigrants. Income is high but US credit history is near-zero, so a mortgage or a rental gets blocked. Don't rush to carry a balance to "build score"—use a secured card / authorized user / 100% on-time and you'll start in months and reach a strong band within a year or two.
High net worth is a high-value target for identity theft. Freeze at all three bureaus first; don't rely on paid monitoring alone. Your name is worth impersonating to open credit—put the defense in front.
Don't disturb your credit right before a big loan. In the months before a mortgage application, don't cluster new cards, hard inquiries, or big charges that spike utilization—it lowers your score and raises the rate on a 30-year loan, a cost that dwarfs any rewards.

Deeper Questions

Does a high credit score equal financial health?

No. A credit score only measures your reliability at repaying debt—not net worth or savings. Someone carrying a lot of debt but paying on time every month can have a high score and still be financially fragile—one layoff from collapse. A score is the price tag on borrowing, not a financial checkup (health is ratios—see Day 1). Don't mistake a high score for evidence of wealth.

Is it fine to skip credit cards and use only a debit card?

In the US, that leaves your credit file too "thin," hurting you on mortgages, rentals, and certain services. The more rational move: use one or two no-annual-fee cards like a debit card—paid in full every month—purely to maintain a solid history, not to spend or chase points. The tool is neutral; the point is that you use it, not that it uses you.

What happens if I close a card I don't use?

It usually nudges your score down: it cuts your total available limit (instantly raising utilization) and, years later, lowers your average account age. Unless the card carries a high annual fee you can't use, keeping it usually beats closing it—put a small recurring charge on it, auto-paid, to keep it active. If you must close one, close a newer card, never your oldest.

This site is evidence-based personal-finance education, not personalized investment / tax / legal advice; consult a licensed professional for your specific situation. The factor weights, score bands, and APR ranges here are illustrative descriptions at a point in time—FICO has multiple versions and credit rules change over time and by institution, so defer to official sources and your own issuer/bureau. US credit and accounts are the default background; China-specific differences are flagged.