The headline indices rise and fall with corporate earnings, global cues and bond yields. For most SIP investors, day-to-day swings matter far less than staying invested. Here is how to think about the noise.
The Sensex falls 900 points and the news calls it a bloodbath. It rises 900 points the following week and the same channels call it a rally. Meanwhile your SIP debits Rs 10,000 on the fifth of every month regardless, and the honest truth is that this is the part that matters.
Still, it helps to understand what actually moves the indices, not so you can trade on it, but so the noise stops feeling like a reason to act.
What the Nifty and Sensex actually are
The Sensex is a basket of 30 large companies listed on the BSE. The Nifty 50 is a basket of 50 large companies listed on the NSE. Both are weighted by free-float market capitalisation, meaning the largest companies with the most freely tradable shares move the index most.
Two consequences follow. First, the indices are concentrated: a handful of heavyweight financial, IT and energy names drive a disproportionate share of the movement. Second, they are not the market. Mid-cap and small-cap stocks, which is where a lot of retail money now sits, can move in a completely different direction on any given day.
The five forces that move the indices
Corporate earnings. Ultimately a share is a claim on a company's future profits. Quarterly results, and more importantly the guidance management gives about the next few quarters, are the most fundamental driver. Earnings season produces the most justified moves.
Interest rates and bond yields. When bond yields rise, the risk-free return available without owning equity goes up, and equities have to compete. Higher yields also raise the discount rate applied to future profits, which mathematically reduces what those profits are worth today. This is why the market reacts to central bank meetings in India and abroad.
Foreign portfolio flows. Foreign institutional investors move large sums in and out of Indian equities based on global risk appetite, currency expectations and relative valuations. Sustained FPI selling can push the indices down even when domestic fundamentals are fine. Domestic institutional buying, much of it funded by monthly SIP inflows, has increasingly acted as a counterweight.
Currency and commodities. The rupee's level affects importers, exporters and foreign investors' returns. Crude oil prices matter disproportionately to India because of the import bill, feeding into inflation, the fiscal position and margins across sectors.
Sentiment and positioning. Elections, geopolitical events, global market moves overnight and simple crowd psychology can move the indices several percent with no change in underlying fundamentals at all. These are the moves that reverse fastest.
| Driver | Typical time horizon | Should a SIP investor react? |
|---|---|---|
| Corporate earnings | Quarters to years | No: this is what you are buying |
| Interest rate cycle | 1 to 3 years | No: cycles turn |
| Foreign flows | Weeks to months | No |
| Currency and crude | Months | No |
| Sentiment and news | Days | Definitely not |
Notice the pattern in the right-hand column.
Why volatility is a SIP investor's friend
A systematic investment plan buys a fixed rupee amount at a fixed interval. When the index falls, the same Rs 10,000 buys more units. When it rises, it buys fewer. Over a full cycle this produces an average purchase price lower than the average market price, the mechanism known as rupee-cost averaging.
A simple illustration. Suppose you invest Rs 10,000 a month and the NAV moves as follows:
| Month | NAV (Rs) | Units bought |
|---|---|---|
| 1 | 100 | 100.0 |
| 2 | 80 | 125.0 |
| 3 | 70 | 142.9 |
| 4 | 90 | 111.1 |
| 5 | 110 | 90.9 |
You invested Rs 50,000 and hold 569.9 units. The average NAV across those five months was Rs 90, but your average cost was about Rs 87.7. The fall in months two and three did not hurt you. It is precisely where the extra units came from.
This only works if you keep investing through the fall. Stopping a SIP during a decline converts a mechanical advantage into a permanent loss of the cheap units you would otherwise have bought. You can model different scenarios in the SIP calculator.
The behaviour gap
Studies of investor returns consistently find that the average investor earns less than the average fund they invest in. The gap comes entirely from timing: money flows in after good performance and out after bad, which is buying high and selling low with extra steps.
The defences are unglamorous:
- Automate the SIP so the decision is made once, not monthly.
- Set an asset allocation you can hold through a 30 percent drawdown, and rebalance annually rather than reactively.
- Check your portfolio quarterly at most. Daily checking measurably increases the odds of a bad decision.
- Write down, in advance, the conditions under which you would sell. Very few honest lists survive contact with a real market fall.
When you should actually act
Staying invested is not the same as never doing anything. Legitimate reasons to change something:
- Your goal timeline changed. Money needed within three years should not be in equity, regardless of how the market looks.
- Your allocation drifted. If equity has run up and now makes up 80 percent of a portfolio you intended to be 60 percent equity, rebalance. That is selling high and buying low by rule rather than by instinct.
- The fund changed. A change in mandate, a manager exit, or persistent underperformance against its own benchmark over three or more years is worth reviewing. One bad year is not.
- Your income changed. A raise is a reason to step up the SIP amount. Many platforms let you set an automatic annual increase.
None of these reasons appear on a news ticker.
A note on index concentration
Because the Nifty and Sensex are weighted by size, a small group of very large companies dominates. If you hold an index fund, you own that concentration. That is a reasonable, low-cost way to own Indian equity, but it is worth knowing rather than assuming an index fund is automatically diversified across the whole economy.
For most investors, a broad index fund plus a flexi-cap or mid-cap allocation covers the gap adequately. Adding sector funds because a sector is in the news is usually a mistake.
The uncomfortable summary
The indices will fall sharply several times over any long investing life. Corrections of 10 percent happen most years, and drawdowns of 30 percent or more happen every decade or so. These are features of equity investing, not failures of it. The return premium equities offer over deposits exists precisely because you have to sit through this.
The SIP investor's entire edge is the willingness to keep buying when it feels wrong to. Everything else is commentary.
What a fall actually costs a long-term investor
It helps to see the arithmetic rather than the emotion. Consider someone twelve years into a Rs 15,000 monthly SIP when the market falls 25 percent.
The corpus drops sharply on paper, and that number is genuinely unpleasant to look at. But three things are true at the same time. The units held do not change, only the price attached to them. The next twelve months of contributions buy roughly a third more units than they would have at the peak. And no loss is realised unless the investor sells.
The investor who stops the SIP locks in the first effect and forfeits the second. The investor who redeems converts a paper decline into a permanent one, and typically re-enters after the recovery is visible, which is to say after it has already happened.
This is why the standard advice sounds so unhelpfully passive. Doing nothing is not laziness in this context; it is the strategy, and it is difficult precisely because it feels like inaction while everything around you demands a response.
Disclaimer
This article is for educational purposes only and is not investment advice. Past market behaviour does not predict future returns, and all figures used are illustrative. Equity investments carry risk of capital loss. Consult a SEBI-registered investment adviser before making investment decisions.