second quarter and asset impairment charges, which are included in salary and service costs, recorded in the fourth quarter, partially offset by an increase related to reimbursements and tax credits under government programs in several countries during the year (see Note 1 to the consolidated financial statements). In 2020, net interest expense increased $5.5 million year-over-year to $189.5 million. Interest expense on debt decreased $27.6 million to $199.6 million, primarily reflecting a reduction in interest expense from our refinancing activity at lower interest rates in the second half of 2019 and the first quarter of 2020, partially offset by a loss of $7.7 million on the early redemption of the remaining $600 million principal amount of the 2020 Notes in the first quarter of 2020 and the interest expense from the issuance of the 4.20% Notes in April 2020 (see Note 7 to the consolidated financial statements). Interest income in 2020 decreased $28.0 million year-over-year to $32.3 million primarily due to lower rates. Our effective tax rate for 2020 increased year-over-year to 27.1% from 26.0%. The non-deductibility in certain jurisdictions of a portion of the COVID-19 repositioning costs recorded in the second quarter of 2020 had the effect of increasing our effective tax rate for 2020. This increase was substantially offset by a lower effective tax rate on our foreign earnings resulting from a change in legislation. In 2019, income tax expense was reduced by $10.8 million primarily from the net favorable settlements of uncertain tax positions in certain jurisdictions. After considering these items, our effective rate for 2020 would have approximated the rate for 2019. Net income - Omnicom Group Inc. in 2020 decreased $393.7 million to $945.4 million from $1,339.1 million in 2019. The year-over-year decrease is due to the factors described above. Diluted net income per share - Omnicom Group Inc. decreased to $4.37 in 2020, compared to $6.06 in 2019, due to the factors described above, as well as the impact of the reduction in our weighted average common shares outstanding resulting from repurchases of our common stock through March 2020, net of shares issued for restricted stock awards, stock option exercises and the employee stock purchase plan during the year. As discussed above and in Note 1 to the consolidated financial statements, our results of operations for 2020 included COVID-19 repositioning costs and asset impairment charges, which were partially offset by an increase attributable to reimbursements and tax credits under government programs in several countries. The after-tax affect of these items on net income - Omnicom Group Inc. was $146.8 million. The impact on net income Omnicom Group Inc. of these actions was (in millions): Net Interest Expense Income Taxes Net Income and Net Income Per Share - Omnicom Group Inc. COVID-19 Repositioning Costs Impairment Charges -  Underperforming Assets Reimbursements and Tax Credits Under Government Programs Total Increase (decrease) to income before income taxes $ (277.9) $ (55.8) $ 162.6  $ (171.1) Increase (decrease) in income tax expense (54.8) (8.7) 39.2  (24.3) Increase (decrease) in net income Omnicom Group Inc. $ (223.1) $ (47.1) $ 123.4  $ (146.8) 19