How spreading risk across companies, countries, currencies, and asset classes works, and why allocation matters more than fund picking.
Diversification spreads exposure across many companies, sectors, countries, and asset classes so no single failure sinks you; your stock/bond/cash split (asset allocation) is usually a bigger decision than which similar index fund you choose.
Diversification means not depending on any single outcome. Across companies and sectors, it protects you if one firm or industry fails. Across countries, it reduces dependence on one economy or political system. Across currencies, it means your wealth is not tied only to the yen. Across asset classes — equities, bonds, cash, and REITs — it blends holdings that do not all fall together.
A broad global equity index fund handles the company, sector, and country dimensions in one purchase. Adding bonds or cash addresses the asset-class dimension and lowers overall volatility.
The split among stocks, bonds, and cash is usually a larger risk decision than choosing between two similar index funds. Moving from 100% equities to 60% equities changes your severe-market loss from roughly −50% to roughly −30% — a far bigger effect than a 0.05% difference in fund fees. Decide your allocation first, based on time horizon and how much loss you can hold through, then choose low-cost funds to implement it.
Three illustrative profiles: a conservative investor might hold more bonds and cash and a minority in equities; a balanced investor a roughly even growth/stability split; a growth investor mostly equities. None is universally correct — the right one is the one you can keep during a crash.
It is natural to over-invest in what is familiar. A Japan-only portfolio concentrates you in one country, a single-currency environment, and an economy heavy in industrials, financials, and exporters. A modest Japan tilt can make sense if your future spending is in yen, but a global core spreads risk across many economies and sectors.
Finally, remember diversification’s limit: in a crisis, correlations rise and most assets can fall together. Diversification reduces the risk of any one holding wrecking your plan; it does not promise a positive year.
Brokerage (NISA/iDeCo) · Rakuten Securities
A leading low-cost brokerage for NISA and index-fund investing, integrated with Rakuten points and Rakuten Bank.
Fees: Many domestic funds and trades are low- or no-commission — verify current fee schedule.
Brokerage (NISA/iDeCo) · SBI Securities
Japan’s largest online brokerage by accounts, with a very broad low-cost fund lineup and full NISA/iDeCo support.
Fees: Many trades and funds are low- or no-commission — verify current fee schedule.
Brokerage (NISA/iDeCo) · Monex
An established online brokerage often chosen for US-stock access and research tools, with NISA and iDeCo support.
Fees: Commissions vary by product; some funds/trades are low-cost — verify current schedule.
This is education, not a recommendation. Most beginners research low-cost, broadly diversified index funds — for example all-country (全世界株式) or S&P 500 trackers — rather than picking individual stocks, because low fees and diversification are within your control while returns are not. Understand that values fall as well as rise, match the risk to your time horizon, and never invest money you may need soon.